Late getting in on this, but bro, sit your sister down and have a serious talk with her in concern for your mom. Your sister is being selfish by ratting on you especially to your mom being in her condition.
Tell your sister the facts. You are you, she is she, live and let live. I would put a major ass guilt trip on sis about your mom. Shit man, put her down, rub her nose in it. That's what she is doing to you.
Brother, alot of respect usually follows one that is one with himself and doesn't allow himself to be the bad guy as in this case.
Give your sister living hell. I've had to do mine the exact way. Scare the shit out of her if you have to. So what. It works. She is so wrong for placing the guilt on you and especially including your mom. God bless your mom. Don't know her but she's in my prayers.
Chin up and walk tall.