How do i loose weight?

Im 14 and swimming in pe is coming soon and i dont want to be the fat one. All we have is unhealthy food and my anorexic and bulimic older sister thinks shes in a competition of losing weight. So she wont give me the key for the fitness room. I especially want my belly slimmed and legs toned and no jiggly fat everywhere. My sisters wont let me get healthy food and when i put the junkfood out of the cart they get mad at me. So two things how do i loose weight? And how can i help my older sister? She 18 5'5 and is probably like 95 pounds. Im 5'4.