Boney back Vs. Thunder thighs

Hello, I'm a 14yrld girl at 52kg and 171cm tall- (that giving a 17.8 bmi) my concern is that I'm at a bmi where I wouldn't want to be any thinner but the problem is I'm really thin in some spots like my spine & hips as the bones slightly stick out however I have big thighs! So the problem is that if I lose any more weight I'd be getting to be on the unhealthy side but if I did I'd be a while lot happier as my thighs wouldn't be so big. Is there anything I can do? Excersises ? Or is having big thighs something I'll just have to live with :/ I'd love & appreciate your help & advice Thank-you