Not sure about this one either.
He sounds a bit childish.
I'd let it go, and not bother with him or your 'friend'.
He can't really care for you if he's vengeful. It sounds like he's playing mind games with you. The beautiful thing with mind games is it take two to play them, so don't play.
As for your friend, people can change dramatically over their teens/early 20's. If you don't trust them anymore I would leave them be, but not make a big fuss or anything over it.
How close are you to your sister?
Is she older or younger than you?
Does she know that you two used to be an item?

Whatever the circumstance is, don't let it get to you. Instead concentrate on doing things that make you happy and add meaning to your life, like sport, hobbies, school or family events. Surround yourself with worthwhile relationships with people who genuinely care.