Ive considered a counsler ( not only for things like this, for my past in general) but ultimatly decided against it because I have a hard enough time opening up to the man Im going to marry about some thing, let alone a stranger.
I know this response is a bit late, and it's good to hear your sister is home, but If I might say something? I too have a "cancerous" family- that is a great way to put it. And Although I always did my best I ended up being kicked out repeatedly after my 16th birthday and had 2 years where I was hoping from house to house. I know you said you don't want to see a consellor, but like yourself I am very very closed even with my live-in boyfriend of 5 years. But I saw a consellor and it has helped a ton! It also wouldn't be a bad idea to try and form a relationship with your sister if not witht he rest of your family. I am still trying to maintain at least a civil relationship with my family when I have to see them but have made it my priority to be in my siblings lives and I have definately prospered because of it. You obviously know your own family better than anyone else on here. But you do only have 1 family, make the most of it. I'm sure you could help your sister tons as it seems like you are a good person just trying to remove yourself from a bad situation so you can benefit yourself.

Good Luck! And I hope she does better!

Cheers.