Am I wrong to be mad about this?

I'm a teen and I still live at home, I'm overweight and told my dad id like to eat healthier. So we've been going grocery shopping for lots of fruits veggies, fish and chicken. My brother keeps bringing his girlfriend over every single day. By the time it is time for me to eat and cook for myself, that girl is just chilling in my brothers room eating damn near all of it, or just all of it. My dad bought something specific for me, and I seen them two cooking it. I told him that it was mine because of weight loss and she just laughed. She's already a very naturally thin girl. Idk why she's over here anyhow. He's known her a month and all she does is yell and wake my dad up when he tries to sleep for work. It's not like she occasionally eats dinner with us, or eats a little, it's almost everything everyday. It's upsetting to me because my trying hard to get the body I've always wanted, and it's hard when you're losing it to the one whose already getting fed and a rich grandpa who she lives with. It's hard when my dad struggles to feed us 4 as it is. (Me, my dad two brothers.)