How could I convince my mum to let me shave my legs?

I am 12 and on my sisters account, so I have really longish-dark brown/black hair on my legs. My sisters is blond and I don't know if she does as we don't share that. I hate wearing shorts/dresses and have to wear tights in any color. I am really sick of this and I am going on holiday for a week in august and I am upset about it. No-one has made fun of me yet, but at my school in the summer term we have to wear shorts but we have knee high socks. I get rid of facial and underarm but she won't let me do my legs. She also says I look beautiful and I don't need it but it is really getting me down. HELP? Thanks to all.