Hopefully your sister isnt harming hun but I guess if she is, at least she has a big sister that really gets it.
I didnt get on with my older brother when we were younger but I always looked up to him regardless, its likely its the same for your sister

Unblocking her on msn sounds like a good idea hun, even if she doesnt take you up on it right now, it will let her know that you're there for her if she needs you
x