Why were they mocking the way I walk. Why is my sister trying to change me?

Don't sweat the small stuff girlie! Some people just get bored with themselves so they choose to mock other people to cure that. As for your sister, she's just trying to "help" you, even though you don't need or want her help. You need to tell her! You DO NOT need to seek approval from her. Before you go trying to please someone else, you have to please yourself. That means buying clothes that YOU want and like and walking however the hell you want to walk! Only do what you want and what you feel comfortable with and yourself will coming running towards you!