When I was a teenager, 17 years old, my family had a ski boat, which we used on many summer weekends and evenings. We lived close enough to the lake that we would often go water skiing for a couple of hours when Dad got home from work in the evening. It was during one of these short, evening ski trips that the experience I’m about to tell you about happened.

While I was part of a big family, Mom, Dad, and five kids, Heather my older sibling was off to college by the time I was 17 and Mom had no desire to ski or put on a bathing suit, so it was usually my other sister Colleen, who was 16, my twin brother Todd, Dad and me (Don) who went water skiing. The youngest Scott was 14 and had no interest in the boat or skiing so he always stayed with mom and played video games.

Ever since I had reached puberty, I had the hots for my younger sister. I think this was more due to her being around all the time than anything else, as she was just plain average looking. She was blond, about 5’4”, probably 110 pounds. But, when you are 17, horny as hell, introverted as I was, even a sister can look tempting.

Not that I had ever got any relief from my sister, occasionally getting to enjoy her soiled panties left in the laundry hamper that I would jerk off to as I sniffed them. She was modest, and I could only imagine what her tits looked like under the tight blouses she wore, and I was mesmerized when she wore jeans and the seam in the back would be up in her ass crack. But I never got to do more than glance at her butt and pussy mound.

I recall once sitting at the dinner table, Colleen on my right, and noticing that her blouse was pulling apart between buttons. I could just see part of her bra and the top part of her breast. As small a peek as this was, it still provided jerk-off fantasies for me for weeks. I still enjoy jerking off.

One summer evening of waterskiing, however, things changed for the better.

Colleen liked to waterski and she was pretty good at it. She always wore a one-piece bathing suit, and most of the time had a beach towel wrapped around her. She never laid in the sun on the beach, but occasionally we would take a break from skiing, have a snack, let the boat drift for a while, and Colleen would sometimes stretch out on the bow. Since my Dad was there, I didn’t even get to ogle her when she caught some rays on the boat. At best, I could just guess how nice her tits were under the suit, and sometimes get a quick view of her butt and crotch, through her suit, of course, when she would bend over.

One evening, later in the summer, we finished our waterskiing. As usual, one of the “twins” as Dad called us, got off at the dock with Dad to go get the truck and trailer, while Colleen and the other boy stayed on the boat to drive it to the trailer. This evening, it was Colleen and me on the boat, and (as usual) Colleen just watched while I did the driving.

After we dropped Dad and Todd off, we waited for them to come back to the boat ramp. Finally, I saw Todd waving from the dock, so we motored back. Todd told us the trailer had a flat, and since there was no spare, he and Dad had to drive back to town to get the tire fixed. We were to just putter around close to the marina for the hour or so it would take. I turned the boat around and headed out. We pulled a mile or so from the marina, and I turned off the engine and let the boat drift. Colleen headed to the bow to lay on the front deck to catch a few last rays. Being later in the summer, the air was cooling down fast, and the sinking sun did not do much to warm things up.

I opened a pop and sat in the Captain’s chair, watching my sister. For once I could stare without worrying if Dad was watching. Colleen was on her back with her head toward me, so she could not easily see me without twisting her head around. Even better, I could see a bit of cleavage down the front of her bathing suit and her protruding pussy mound. Not much, mind you, as the suit was fairly high-cut, and Colleen would be a small B-cup, but it was enough to get a 17-year-old boy horny.

As I sat in the Captain’s chair, I stared at that bit of cleavage and mound, and slowly started massaging my cock through my bathing suit. Colleen could not possibly see this without sitting up, so I felt safe. It didn’t take long for me to get rock hard, and soon the liner of my suit was becoming too tight for my prick. Not that I’m all that well endowed, but I was probably 6” long even then. So I pulled my suit leg and liner to the side, and let my cock out, up it popped hard and ready. I slowly stroked it, occasionally massaging my balls, and soon I was dripping pre-cum from the tip of my prick which I would wipe off with my finger then lick my finger clean. After a few minutes of sitting on the front deck, my sister must have started getting chilly, because soon I could see her nipples poking up through the swimsuit material. This made me start stroking faster, still confident that Sis was oblivious to my staring and stroking. A couple of times I approached orgasm, but I would stop for a minute or so, just to keep the pleasure building. Besides, how often could I stare at my sister’s tits and nipples let alone her pussy mound, even through a bathing suit, for an extended period?

After perhaps 5 minutes of stroking, pausing, stroking some more, I was getting near the point of no return. Then I saw my sister reach up and scratch the side of her tit. I’m sure this was done without thinking that I might be watching, but as she scratched and her boob bounced back and forth, I hit the point of no return – I started cumming, shooting white streams all over, and groaning with pleasure. At that young age, I had the (now envied) ability to shoot my cum quite a distance, and this time one gob flew the three feet or so to where Colleen was laying, landing on her swimsuit near the top of her tit. I don’t know whether it was a gob of cum landing on her or my incoherent groans that she noticed first, but, in any case, she bolted upright and turned around, just in time to see the last spurt of cum fly out of my prick.

Caught!!! I knew I was dead meat – Colleen sat there, mouth gaping in surprise (or horror), looking at my wilting prick in my hand. We probably stared at each other for ten or fifteen seconds before she said anything.

“What in the world are you doing!?”

“I’m ... I’m ... well ... sorry, Colleen...”

“I guess I can see what you are doing. Were you jerking off watching me?”

“No, I was ... ah ... I was ... well, I guess...” I was still incoherent and embarrassed, and I started to tuck my prick back into my swimsuit.

“Hold it right there! Leave your boner out until we settle this!”

I don’t think Colleen knew what she wanted to do right then, but just needed a few moments to think things through. “But grab that towel and wipe up your mess!” I grabbed a towel, wiped up the floor and the dashboard where I had splattered cum.

When I finished, Colleen said, “You missed one spot.”

The only spot I could see was the one on her swimsuit, and I was in enough trouble without going after that one. Colleen turned around, slipped off the front deck and came and stood by me. “If you’re going to make a mess, you have to clean it up! Now wipe off this spot before my suit gets stained.”

I was still immobile from fear, slowly turning into lust. Colleen grabbed my hand with the towel, raised it to her boob, and wiped the cum off her suit with her hand on top of mine. I swear to God that she rubbed my hand back and forth much more than was necessary, massaging her tit with my hand, and giving me another raging hard-on.

As Colleen let go of my hand, she looked down and saw my very hard prick saluting her. “My, oh my, you are hard again already! I think my older brother has the hots for his younger sister! Do you find me sexy, Don?”

Of course, I’m still nearly speechless from embarrassment and in a bit of shock that my sister has not totally freaked out at my cumming on her. Slowly I mumble, “Well, kind of.”

“What do you mean, ‘kind of’?”

“Well ... when you have your bathing suit on, and I can see your shape, I kind of like that. And your n ... ni ... nip...” I still couldn’t bring myself to be this candid. “My nipples? Are you saying you noticed my nipples sticking up through my suit? And that turns you on?”

“Uh, yea and your pussy mound” I sheepishly said.

“Well, I think I’d better tell Dad he has a pervert for a son. I don’t think he’ll be very happy knowing you were jerking off over your sister, let along that you came on her.”

“No, please, Colleen, I’m sorry ... I’ll never do it again ... Please don’t tell Dad ... I’ll do anything for you if you don’t tell Dad!”

“Well, anything can mean a lot of different things.”

“I know ... Colleen, I’ll do dishes on your nights ... I’ll stay out of your CDs ... anything you want ... Just don’t tell Dad!” By now, my raging hard-on had shriveled into nothing and was back in my suit. The thought of my Dad knowing what happened had totally wiped my feelings of horniness away. Obviously, Colleen knew this scared me and was perfectly willing to take advantage of it.

“Well, Don, I don’t really care about dishes and CDs right now. However, since you have agreed to do anything, you can start by taking your suit off so I can see your cock.”

Of course, this was totally unexpected, so I just stared for a few moments. “Take my suit off?”

“Yes, pull your suit off. If you are going to jerk off watching me, I want to watch you, so take off your suit, and rub your prick to make it hard again.”

I slowly dropped my suit but didn’t really need to stroke it, because the thought of my sister looking at my prick had me hard again by the time the suit hit the floor of the boat. Besides, by the time I had straightened back up, Colleen had reached out and grabbed my prick and started stroking it slowly.

“Wow, that got hard fast! Does this feel good?”

The feel of my sister’s hand stroking my prick was incredible. This was the first time a girl had ever touched it, and I think I would have exploded in about ten seconds if I hadn’t just cum a few minutes earlier. “Yea, that feels great!”

I don’t think my sister had ever had any sexual experience with boys at this point in her life. She was only 16, and plain looking, and, as far as I know, had never been on a date. So, perhaps her response to my voyeuristic jerking was in part her own sexual drive and lack of release, or even just curiosity about boys. In any case, I wasn’t complaining.

Colleen sat down on the opposite chair, while I was still standing. This put her face just a few inches from my rock-hard cock. She continued to stroke it slowly, sometimes stopping to squeeze or shift her hand, while looking intently at my prick, in an almost stupor-like fascination. After a minute or two, I started groaning, as it was feeling so good. Colleen looked up and smiled, then took her other hand and started feeling and massaging my balls.

“I’m going to cum again!” I said.

“No! Not yet! I don’t want you to come until I have finished playing!”

Playing? Maybe she thought this was “play,” but it was mind-blowing to me. “You’ve got to stop if you don’t want me to cum!” I finally spouted.

Colleen moved her hands to my belly and thigh and watched closely as my prick twitched in excitement.

“Well, then, perhaps you need to pay attention to me so your weenie gets a rest.”

I don’t know why she calls my boner a “weenie,” but at that point, I wasn’t about to complain.

“What do you mean, ‘pay attention to you’?”

“I’ve made your boner feel good, now you need to make me feel good. You can start here,” with which she slipped off her suit straps and lowed the top, showing me in an instant those two tits with puffy nipples I had been jerking off to for ages.

I was back in stammer mode, “I ... uh ... I ... uh ... well ... what do I do... ?”

Colleen just grinned at my discomposure, put her own hands on the sides of her boobs, lifted them up a bit, and said, “Why don’t you feel these for starters!”

I didn’t need another invitation – I leaned down and put my hands under her tits, lifted them a bit and started massaging them. I had never imagined boobs were so soft! Soon I was running my thumbs around her nipples, which started to harden, and Colleen started to moan a bit.

“Lick them!” she moaned.

At this point, I had to kneel on the floor and tentatively started licking her nipples. After a minute or so, she told me to suck, so I started doing that. I guess I must have sucked too hard, as at one point she exclaimed “ouch,” and pushed my head away.

“Sorry, Colleen, I didn’t mean to hurt you!”

“You’ve got to be gentle!” (Haven’t I heard that from women a few times since!) “Here, let’s go sit where it is more comfortable.”

Colleen stood up and walked to the back where there was a bench seat. She started to sit down, then stopped and said, “I’ve seen all your goodies, do you want to see mine?”