I'm thin, but my relatives aren't?

Ok, so I'm pretty thin and healthy (14 years, 5.1, and 105 pounds). I eat healthily (no wheat or gluten for medical reasons). Though I'm thin, my relatives aren't. My parents are pretty skinny, but practically anyone else I'm blood related to weighs... a lot. One of my relatives was super skinny and gorgeos when she was my age and in high school, but now she's overweight. I'm scared that these genetics will be passed onto me. A couple weeks ago I found some cellulite which is just gross. I think this is from genetics because I'm not overweight or anything. Basically what I'm asking is is there any way to beat out genetics and get toned and possibly lose some weight?