Cutting off my Mother?

To start this off, I’m 20 years old, full time college student, part time retail associate. My relationship with my mother has gone awry ever since my dad died in 2011. She’s become increasingly toxic and possessive with me and it’s starting to be too much. Bottom line is, I’m in a relationship now and she always gets mad at me for going to spend time with him. Mind you, I only see him once or twice a week. Any other spare time I have is spent at home. We currently live with my older sister but my mom is ready to move out, however due to escalations in the negative aspect of our relationship, I no longer think living alone with my mom is a good idea. My sister said I can live with her, how can I tell my mom I’m not planning on living with her. I already know it’ll be nothing but arguments.