How can a pre-teen (12) loose weight?

[Korrtni]

hi. this is my first question so i dont really know how this works. anyway, my little sister is 12 and she's very insecure about her weight. she wants to loose weight, but doesnt know the best way to do so. she weighs more than me and my mom (180lbs). she doesnt look as big as she is because she is tall for her age 5'5. she lost weight before, but gained it all back. we've asked for my mom to buy less junk foods, so my sister wouldnt give into temptation. well my mom still buys them and it makes her hard to resist... plz help thnx