I'm SO worried about my little sister?

my little sister is 7, and she already weighs about 81 pounds! my family has genes for being born/ easily prone to getting obese or overweight. so my family is overweight, but i'm extremely worried about her. she over eats... really bad. like whenever my grandmother gets home from grocery shopping, the first thing she does is go straight for the cookies, candy, and chips. we will get a family size bag of doritos and in 2 minutes half of it is gone. and whenever we have spaghetti, she will eat lots of it, really quickly. then have like 2 plates of it, and about 7 bread sticks! plus i'm a vegetarian and i have these veggie nuggets(ik weird but they exist) and she kept trying to steal like 3, while i had like 6. she snacks A LOT too! so bad, i have to hide my healthy snacks and my school lunch snacks so she won't eat them. i am trying to lose weight and i'm 13, when i was her age i over ate a lot also. i over ate my self to 181 pounds at only 11, i found that out the hard way that i needed to change. I made the decision to lose weight and become a vegetarian because my mom and grandma let everyone eat anything at any time. but i became determined and i'm 143, only 26 more pounds to go. i feel so bad for my sister because it's so hard for her to find clothes she likes that will fit her. i need to cry every time i see her snack and over eat. by the time she is my age, i will be in college and not there to stop her and tell her what will happen. because she may see her weight as ok and eat herself to obese! i am so worried for her, she may not be as lucky as i am to see the right path. HELP?! :'(