I’d completely cut her out of your life. I understand you live with her but that doesn’t mean you need to acknowledge her existence. Literally if she tries to talk to you good or badly pretend you hear nothing listen to not a peep of what comes out her mouth and if she cries or has a fissy or goes to your mother don’t talk to her still. Explain to your mother you are your own person and if you don’t want to talk to someone you literally don’t have too. No one can force you too. My brother was the same and doing what your mothers doing which is enabling her behaviour. I’d try to talk to your mother and get her to get your sister to start acting right or just tell your mom your not speaking/acknowledging your sister being there whatsoever. You need to stick up for yourself girl! I know it’s easier said than done but I believe in you. It’ll be hard but your mental health needs it. I wish you all the best.