My Friend, her crush.... AND HER SISTER!?

why would you need to do anything? it's between your friend and her sister. the thing you can do is be there for your friend, be understanding and listen to her when she needs someone to talk to but she has to take care of this one on her own. i know its hard, you want to be there to help her. but the best help you can give her is to let her handle it and be supportive in whatever happens. sometimes being a friend is the best help you can give.