A great start by a new author.
Stampedicus is new here at StoriesOnline, but I'd have to say that he's off to a good start.
The story has a number of things going for it. The overall premise of an older sister looking out for a younger is reasonable and the rationale presented is logical in the context.
I thought that the dialogue sounded realistic, and in general, getting dialogue right is something that a lot of authors struggle with. So full marks there.
There's a bit of character development, probably about right for the length of the story. It's enough that I was able to identify with the main character to some extent. This is important to me since it makes it easier to get involved in the story.
He's managed to avoid the cliché descriptions of "bounteous boobs" or other gratuitously off-putting descriptions that objectify the female character. This makes it easier for me to imagine myself "thinking along" with what the protagonist is thinking. Having been in bed with one or two women over the years, a story doesn't have to be overly explicit to be a turn-on, but it needs to have some uniqueness that conveys the idea that the experience is special for the characters, and the author has done this.
And, finally, the technical quality of the writing is such that I cannot think of any problem that I noticed. This is extremely unusual. The fact that I did not have to jump out of involvement with the story to deal with technical lapses most definitely contributes to the "flow" of the story. In turn this means that I enjoyed it more.
So, have a read, and join me in hoping that we'll see more from this author.