I understand completely. My brother is a recovering addict. I have often gotten into fights with my mom about wanting to keep my distance and protect myself from his path of destruction. It was extremely difficult to make her understand that I did not hate him, that I loved him deeply but I could not bear the pain he brought into my life and because of this I had to keep my distance and needed to stop "becoming addicted to HIS problems". You have to understand that she is her Mother too. She is still willing to take all of your sisters pain because she is her mother. It helped when my mom actually started to see a therapist about my brother and the best way to seperate herself. I went to a session with her and it really helped her understand my point of view. Yes we still disagree about how best to handle his situations but at least know we are open about it and our opinions aren't held against each other.
*Never feel bad about telling your parents that you need to protect your life/feelings from your sisters behavior. That is your right. You deserve to find peace in your life.
I hope you find peace. I'm still seaching.