AITA for calling my sister fat?

Ok. I know it sounds like I'm obviously the asshole here but listen to the backstory. So I'm the youngest child and my sister is the middle child and because of this she always stereotypes me and calls me the favorite child a lot. For example when it was my 11th birthday my mom took me to chick fil a on my birthday. I love chick fil a but my sister hates it. So the whole time we were driving there my sister was hitting me and my mom, crying, and calling me the favorite child. The worst part of all this is my parents always defended her cause of her "bipolar disorder" that's not even diagnosed,she abuses me and beats me up just for trying to hang out with her and my older brother, and she always body shames me for being skinny (I can't control it cause I have a fast metabolism) and has made fun of my small boobs (that I also can't control) since I was eight. One day at a thanksgiving dinner I had five plates of food since I haven't eaten all day and here comes my sister. She says "omg how are you not fat? You're eating so much food." I roll my eyes and try to ignore her but then she says "all that eating is gonna catch up to you one day. At least I'm trying to lose weight." or something like that I can't really remember. And I said "at least I don't have to try" then she starts crying to our parents, and my parents are pissed at me cause she twisted the story, saying I fat shamed her for eating. I got grounded for two weeks and since I was in that rebellious teenager phase I thought it was worth it cause that was the best comeback in history, but now a few years later it's keeping me awake and I can't help but feel guilty. So, AITA?