Hello Catherine25,
Reading your post, I felt like you were describing my situation to a T, wow. It is so devastating to come to this conclusion, that you have to disengage, when actually what you want most would be a close relationship with your sibling.
I don't have any particular advice, beyond of course, take care of yourself, stay close to those who are very supportive so you don't feel too lonely as you experience this loss.
What's tricky I find is how to manage other family members' responses as you disengage and back away. They should understand since they've also witnessed these rages and have been on the receiving end of the unacceptable behavior, but not everyone is always on the same page when it comes to cutting ties.
In my case, I have maintained contact, letting my sister initiate and responding when she does, keeping it mainly superficial. My therapist explained "dosing" to me -- Just hanging with her in small doses, which seems easier for me than completely cutting off. I also don't find engaging with my sister to be productive whatsoever; trying to reason with her gets me nowhere and actually exacerbates the bad mood. Seeking help through these support groups, friends and a therapist is also great.
Wishing you strength
,
-Pumpkin