MY STEPMOM IS TRYING TO MAKE ME GAIN WEIGHT - HELP ME!!!!!!!!!!!!!?

My stepmom & her daughter are very overweight (I mean, like 300 pds). My dad was super skinny when he met my stepmom & now he's gained a lot of weight. Not like 300 pds, but he gained a good amount. So my stepmom is jealous of how thin I am (I weight 138 pds, but am trying to get down to 105) so she is trying to make me gain weight. Every time I visit, she makes stuff like cookies, brownies, pumpkin pie, & then sends me home w/ the leftovers & she even told me 1 time, "to eat it all night long". For Father's Day, she cut me a piece of cake w/ the most icing & told her daughter to give that one to me b/c it has the most icing. I like seeing my dad but she is getting on my nerves. I used to refuse to see her, but then my dad refused to see me so I was forced to see her. But I don't ever get to spend time w/ JUST MY DAD. She tags along EVERY single time. When my dad picked me up from taking the ACT, she tagged along. She always comes along. Every time they ask me to go somewhere, it's either over their place, where she cooks or out to eat at restaurants I love. I really don't know what to do in this situation. I am not going to stand here & tolerate this & let me cause me to gain weight just b/c she can't control her jealousy. Not only this, but she also acts like we are bffs like she txted me asking for us to go get our nails done like wth? What should I do? and this is a serious question - no joke. Thx - 10 PTS!