He claims the girls are in highschool and that he is going into 6th grade.
So it's my understanding he's ~11 and they're anywhere from 14-18?
I don't think I have any advice for this situation (implying that I give out good advice, ahaha) other than the next time she's around and your sister isn't (because that'd be pretty weird, not gonna lie), do whatever the hell you want. If she pulls your pants down again, leave 'em down, don't run, and see what she does.