Well, doing aerobic exercises (like running long distance, which keeps your blood and oxygen levels fairly consistent) that involve your thighs can burn the fat without building to much muscle, whereas anaerobic exercises (like sprinting. Just getting your heart rate up very quickly) builds more muscle. Try and stick to those longer, enduring exercises to tone up your thighs.
More importantly, eat right! You’re not going to lose anything if you don’t clean up your diet.
Your mother is right about the genetics. Everyone is born with different bone structures and you may not have the ability to thin your thighs down as much as you want to. However, you will be able to thin them some.
Good luck! xx