Going back to Abi’s panties, I think that it was the following day on the bus after all the other kids had got off, when we were talking about, rather than engaging in, incestual activities, that I told Abi that I thought that it was amazing that she loved looking at, tasting and eating the dirty cream which often collected under my foreskin, but wanted her to know that I loved two things as well. She looked intrigued and asked me what they were.

“Firstly,” I said, “I love seeing piss stains on the crotch of your knickers, and I would love you to take your panties off and to then watch me suck those dirty piss stains clean”. Abi bit her lip and said “Oh, yes, that’s very dirty!”, I continued, “Secondly, I really love seeing and slurping up the white cream which sometimes oozes out of your vagina when you get particularly excited; it feels amazing to lick, taste and eat your dirty vaginal cream too”; Abi shuddered and said, “Oh, you are a very naughty brother, aren’t you?”.

She looked around to double check that there was nobody else on the bus, then Abi swivelled in her seat and put one foot up onto the seat cushion, exposing her panties. She looked down and I followed her gaze; the crotch of her standard white school specification panties had a deep yellow stain about the size of a 50p piece right where I knew her sisterly vaginal opening was. She looked up at me and said, “Will this do?”. I nodded.

We were on the top deck of the bus, so Abi stood up and turned round to face me, then she lifted her skirt and rolled it up tightly before grabbing one of the seat backs to keep herself steady. I looked up at her and she said, “Why don’t you pull my dirty panties down brother? You know you want to!”. I knelt down on the floor of the bus in front of her and gently grasped the waistband of her panties and began to slowly pull them down. Abi moved her legs slightly apart as I did so, so that they didn’t bunch up. As they slid down, her lightly fluffy vagina came into view, the little slit tightly closed, but with the hint of the swelling of her clitoris at the top of it. I kept on sliding the panties down until they were halfway down Abi’s thighs, then Abi put her hand on my head. I looked up and she said, “Let’s see those piss stains together”.

She spread her legs a little further apart and I lifted up the crotch of her panties to show both of us the piss stain. “Why don’t you look up at me so that I can watch your face as you lick the dirty yellow piss stains in my knickers for the first time?”, she said. I nodded and stretched the gusset so that I could bring it to my mouth, whilst still looking up at my little sister. I was looking into my young siblings eyes as I slurped at the piss stain in her dirty panties, with her looking down at me, looking very excited as she said “Wow, that is so very, very dirty brother!”. She looked up and saw that we were just approaching our stop, so she quickly pulled up her knickers and dropped her skirt, then we had to get off the bus.

As we were walking home she asked how I felt. I told her that I could still taste her piss in my mouth and that I loved it. Abi grinned and said, “Piss is very naughty isn’t it?”. I agreed. “Did my piss stains taste very rude?”, she went on. “Oh yes,” I said, “very rude indeed”. “Right,” said Abi, “would you like some more tomorrow?”. I gave her a big grin and said “Oh, yes please little sister!”.

The next day as we sat down on the bus after school, Abi squeezed my hand and passed me something soft and white. I looked down at it and unfolded it to find that it was a pair of knickers. Looking around to check that nobody was watching, I unfolded them in my hands and spread the crotch out with the inside of the gusset where Abi and I could both see it; the gusset and the cotton surrounding it were all a very dirty looking yellow colour, and running down the middle of the gusset was the biggest smear of vaginal cream I’d ever seen, some of it dry around the edges, but some still obviously damp. I looked at Abi and she whispered “How’s that?”. “Amazing! How did you do it?” I whispered back.

Abi grinned and said quietly into my ear “I went to the toilet at mid-morning break and didn’t pull my panties down while I had a wee, then I took my knickers off and put them into my locker to dry out. Then at the start of lunch hour I retrieved them and went back into the toilets again and sat there for a whole hour playing with my vagina, sticking my fingers up myself and thinking about all the lovely dirty things we do together, and by the end my vagina was just oozing loads of that dirty white cream you like so much, so I then scooped it all up with the crotch of my piss-stained panties and put them back into my locker to dry. Do you think that was very perverted of me?”, she said, doing her best to look like the innocent 14 year-old schoolgirl she wasn’t. “Very rude!” I said grinning back at her.

Once the last of the other kids got off the bus, Abi sucked my penis, watching as I first licked up the still-wet vaginal cream and then as I soaked her gusset with saliva, sliding my tongue along the dried parts, really enjoying her seeing me get the dirty taste, then I sucked the whole gusset into my mouth and soaked it in more spit, delighting in the mixed taste of my sister’s piss and her juices dancing across my tongue. I soon spunked into Abi’s smiling mouth, which she really enjoyed, swallowing her brother’s spunk straight down.

“Right,” she said, “the next time I stain my panties I want you to watch as I lick and suck them clean myself. How would that be?”. I grinned, and we laughed the rest of the way home.

One day about a week later on the bus on the way to school, before anyone else got on, Abi and I were talking about dirty things again. Abi went into her school bag and pulled out a pair of panties, then she said, grinning, “I had these on yesterday and I had a little accident”.

Abi showed me the gusset of her knickers and there was quite a large piss stain and some dirty dried up vaginal cream on them too. “On the bus home tonight, I want to see what my piss tastes like, so I am going to let you watch me lick my own dirty panties, then I am going to soak them with spit and suck all of that dirty piss out of them.” she said. I nodded and said “You are a very perverted little schoolgirl Abi, sucking your own dried on dirty piss is very rude, but doing it while your older brother watches is very perverted indeed!”.

Abi asked, “Will you lick my dirty under-aged vagina out while I’m licking my own dirty panties clean please brother?”.

Normally I’d have jumped at the chance to slurp my young sister’s incestuous 14 year old vagina, but I’d been thinking about how dirty and delightful the taste of Abi’s panty piss stains was, and I’d come up with some new dirty things that I thought Abi might like to try.

I shook my head, and she looked a little taken-aback, so I quickly said “Nope, while you’re licking you own piss-stained panties clean, how about we do sometinhg new and dirty. I want you to piss into a glass that I am holding?”. Abi’s mouth dropped open. “Oh yes!” she said, “That will be so dirty and exciting! How will we do it?”.

I said, “There’s a double-decker bus on this afternoon, so why don’t we go and sit upstairs, then when everybody has got off the bus you can squat down on the back seat, then while you are licking your own piss-stained panties, I will kneel in front of you holding your vaginal lips open so that I can see your little piss hole, then I want you to have a dirty piss into a glass while we’re both watching”.

“Oh, that’s sounds wonderful brother!” said Abi, “That sounds like a really dirty thing to do, doesn’t it?”. I nodded, then Abi said, looking disappointed, “Oh, but we haven’t got a glass!”. “Don’t worry,” I said, “I’ll pinch the one I have at lunchtime at school and pop it into my school bag, then I can take it back tomorrow.” Abi let out a chuckle, saying, “You have a very perverted mind indeed my incestuous dirty brother! Oh yes, I’d like to see you watching my little piss hole spurting dirty urine into a glass!”.

A thought struck Abi, and she said, “But what will we do with the dirty piss once I’ve spurted into the glass?”. I’d already thought about that of course, and replied, “Well, I already love the taste of your dirty piss and by then you’ll have sucked your own dried on piss stains off your own panties too.”, she nodded enthusiastically. “I just know that you’re going to love the taste of your piss as much as I do, so why don’t we both have the very dirty thrill of drinking hot dirty schoolgirl piss straight after it’s been spurted out of your under-aged vagina?”. Abi grinned, and said, “Oh yes, drinking dirty hot piss sounds so very dirty and depraved!”. “You only need to spurt a little bit of piss into the glass if you want to.” I said. Abi nodded and said “This is going to soooooo naughty!”.

That evening, my ‘innocent’ little sister and I had our first experience of proper dirty piss sex. She came out of school with no knickers on, and as soon as the bus had emptied I had the really dirty pleasure of spreading her little vaginal lips open as she squatted on the back seat of the bus as she showed me the gusset of the previous day’s yellow piss-stained panties again. She got some saliva in her mouth, which she then dribbled onto the dirty stains, then she let it soak in, before sliding her tongue over the stains. She looked up at me and said “Wow, that tastes so rude!”. She looked coy for a second and then said, “This is just like me licking my own vagina clean after I’ve been for a wee, isn’t it”. She shuddered and then she pushed the stained gusset into her mouth, obviously salivating as she began sucking the juices out of it, making very dirty slurping noises as she did so, with a very dirty look on her 14 year-old face.

I couldn’t help giving her very wet vagina a few licks to get the taste of her under-aged hole, then I held the glass close to her little vaginal opening and spread her lips with the fingers of my other hand, as she held onto the backs of the seats with hers, still sucking the piss juice out of the panties in her mouth. She took the panties from her mouth and said “Are you ready you dirty boy?”; “Oh yes Abi, I’ve been looking forward to seeing you piss all day!”. Abi grinned, popped her perverted stained knickers back into her mouth and closed her eyes, and then seemed to press down inside. A spurt of hot schoolgirl urine shot from her little vaginal opening and into the glass as she opened her eyes to look down to see her own brother not only watching her committing such a usually intimate act, but also to see her own dirty bodily waste collect in a school water glass.

Abi ‘s urine was quite dark, and it looked like she’d been saving it up, as it was spurting with some force from her little piss hole, making piss foam as it flowed. I thought she might only piss a little into the glass, just for us to get a taste of it, but no, Abi just kept on spurting until the glass began to get full. Abi was moaning with passion as she took in the scene in front of her with the taste of piss and vaginal juices in her mouth, and I was panting with lust too. With an effort, Abi stopped the flow, although dribbles of hot schoolgirl piss continued to dribble down the inside of her thighs. She took the panties out of her mouth and said “That was so rude! I just loved it!”. I grinned back at her nodding my head. “We’ve only got ten more minutes, so let’s drink my piss while it’s still hot!”, she gasped. I handed the glass to her and she put it up to her nose; she sniffed deeply and said, “That smells so wonderfully perverted! Going to the toilet will never be the same again brother”, she said with a wickedly perverted leer on her lips.