I am so sorry.

Yes, tell her, but take your time with it and don't feel you have to get it all out in one shot. Your sister's illness and death, by itself, is enough to have thrown your family into chaos and set you up for a lot of pain and vulnerability to be used by others.

I'm glad you're taking a step to resolve this. I've read your posts for some time and know that you're in a lot of pain. Be well.