I could never fuck a girl any younger than my sister (17), that shit would just creep me out. You gotta think, every girl (ok not every, but for my point...) is someone's sister.
On the other hand, I wouldn't care if any of my friends was with my sis cuz she's goddamn old enough to make her own decisions, and she'd tell me if there was a problem with anyone. Lucky for me, she hates most of the guys that try and get with her.
Funny story: family vacation in the British Virgin Islands, we're getting drunk and hangin with a bunch of people at this bar, and this 40-somthing dude comes up to me and says "man, you're pretty lucky to be gettin a piece of that" and points to my sister. it wasn't like we were hangin off each other, we just walked in together and were playing pool against a honeymooning couple, so the guy just assumed. Instead of jacking him in the face like 90% of my friends woulda done, I just told him straight up "dude thats my sister". So he paid for all my drinks the rest of the night.