Would biking get legs skinnier?

You see, my little sister is 10, going on 11 on the 7th of July. She keeps on nagging me how am I so skinny and stuff. She thinks she's fat, but to be honest, I believe she's a beautiful child. But she keeps on saying "I'm fat". I'll be honest, she's beautiful, but her thighs aren't the best thing around, but from her stomach to her head, is like, looking in the mirror. She adores biking, but she only bikes in the summer. You see, I don't know how to measure legs with a tape measure. But if I put the tape measure around her thigh, it's 17 inches. In her class, the girls are pretty darn skinny. She really wants to be skinny by September 6th or her Grade 6 shopping spree, (Which is the last week of August) I'll have to admit, she's pretty young to start losing weight, but she wants to get skinnier, and thats her wish. Will biking get her skinnier?