What do I do if my younger sister goes to my prom with my friend?

As idiotic as she is going to make you look its your responsibility as her brother to attend to her well being. That is – warning her about your friend and his likely intentions towards her. Even if she laughs it off, or finds the idea extremely odd or unbelievable. If you do feel that he is in fact getting into her ‘’pants’’ then I suggest you do everything in your power to stop him. After all your sister is in fact very young and doesn’t need this from a guy. As for the prom, just keep a close eye on her. If things get suspicious then I would suggest you look into it to make sure. If things do happen to get out of hand, then leave.

you don’t understand girls !! you’re her brother and you worry ‘bout her but that’s her business !!! You gotta pay attention but don’t meddle with her business !!! I know you’re brother but … when you have relationship with any girl you think ‘bout it that you’re sister is GIRL too !!! you gotta think ‘bout it !!!

How exactly is your sister going to make you look like an idiot? Just go… And your sister is 16. She’s not 5. Warn her about him. And then realize that you cant do anything more than that. If she chooses to have sex with him, that is her choice.