Sister saying hurtful things

My sister is always talking crap about me and I'm honestly tired of it. She will talk about me to my niece. She told my niece that if she keeps eating how she does that she will get fat like me. My niece is very close to me and she tells me every time her mom says something to her about me.. My sister doesn't know this bc I never say anything to my sister. Her saying that really made me upset. Yes I'm overweight but I have lost 20 lbs bc I'm Trying to get healthy for my kids and myself. I didn't know my sister was talking crap about my weight but now that I know she's said this to my niece who else does she talk crap about me too.. She's told me I'm too sensitive, she's called me selfish for choosing not to watch her son bc he doesn't listen, and she's also said stuff about my love life.. I could live with my life happily without ever having my sister around again and it shouldn't be that way. How do I deal with this? Also she's very insensitive so talking to her about this is useless bc she won't give a crap about what I have to say or if she even hurts my feelings. My niece is 14 btw and I did tell my niece that her mom shouldn't be saying that stuff to her. Also I've been overweight since I was a pre teen bc I was bullied often in middle school and turned to food for comfort and my sister knows this. My niece is very sensitive and so her feelings get hurt quite easily. I knew my sister said crappy things to me but I didn't know she said stuff like that to my niece.