advice/venting

So before my dad got married why now set brother thought i was cute. Well fast forward 2 yrs he’s 17 i’m 15 and he still thought it and we were talking a lot Well we hadn’t talked in a few days due to being busy but last Thursday (it’s Wednesday now) we were in his sisters room and she’s also 15 (5’7 130lbs i’m 5’10 160) and he made a comment on how “with you stomach and body you’re the type boys aren’t gonna like) I really hurt me and I thought as time went on it would get easier but it’s gotten harder to the point i at first cried like every night and now every time I got to eat I start hearing that said and i start crying. Last night his other brother bought us (me him and his sister) McDonald’s and after crying off and on for an hour I gave it away. I know it sounds like a stupid reason but It’s rlly messed me is because it was said by someone that says they love me and care about me and in my head if that’s true why would they lie so then i believe it. I already knew i wanted to lose some weight but now i have the urge to lose a lot. I’m worried ab falling into an easting disorder with where my mind goes when I try to eat. I rlly would like your advice. I can’t go to my dad or step mom because they don’t understand and just say “you aren’t fat” or get mad and say “you’re fine, eat”. And little “harmless” comments get said jokingly but over time as they build up it hurts. I know it’s a long post I apologize.