Dbh, I am so very sorry for the loss of your beautiful sister.
Nothing you did or could have done would have changed her actions. Someone I love very much made several serious attempts on her own life. She has said that it is like being on a train - there is not anything we could have done to prevent her. We'd have been run down in the process. She was found and the immediate crises taken care of medically. Then came years of gut wrenching personal work. She chose to do that. If she hadn't, she'd be gone now, I have no doubt. She still struggles with the dark thoughts but she tries very hard to not get on the train.
Suicide is cruel to everyone it touches. Please know that her pain was such that she was incapable of seeing how it would hurt those who loved her. And please know that you do not have responsibility for her choices. Take care of yourself during this awful time as best you can. If that means distance from your mother, so be it. She is lashing out with her own pain, from her own unwell state. But her pain is not your burden. You have enough of your own.
Praying for you and your family. I am so very sorry you have to go through this.