I'm glad to hear that the other members of the fraternity did not try to make excuses for that brother, and that they took it upon themselves to apologize for him.
Your sister's statement about the skirt, on the other hand, is completely uncalled for! How dare she!
I guess she also is of the opinion that any woman who gets raped "asks for it."
I would definitely say something to her about that comment, and let her know that it was unacceptable and totally out of line.