My prayers are with you and your family. I have walked that road before, and the decision to leave is never an easy one - sometimes precisely because we want to think we're stronger than that. But abuse is abuse, and should not be tolerated, especially when children are involved.
You are doing a fantastic job of supporting your sister thru this very difficult time
If not for my church (I'm an only child and lived far from any family) I am sure my separation would have been impossible for me to deal with. Seniority is NOT a consideration in their eyes: encourage her to talk with her pastor about her needs. I have every confidence they will be met and exceeded.
You are doing a fantastic job of supporting your sister thru this very difficult time