(((Spalding))) So sorry to hear this. Hopefully your mom will do what she needs to do to get healthier and get on the transplant list if that's what she needs.
As far as telling your sister, can't you just tell her to ask your mom if she has any questions? It seems like an unfair burden to make you the go between, when it's really not your responsibility.
I'm really glad you're getting support from a counselor right now. Caring for yourself is crucial. As far as bearing a burden of guilt, you can make amends to your mom if you wish. But also remember that for years she has chosen to drink even when it had a negative impact on her parenting abilities, relationships and health.
Keep reaching out and posting. We are here for you.