My parents prefer my sister

I'm 17 and my parents prefer my younger sister who is 14. they always say she is beautiful,clever and that she gives them a lot of satisfaction and instead they don't say anything about me ! when I and my sister have an argue they always say she is right! once I and she went shopping and she bought a pair of shoes she was not allowed to buy because it was too expensive, I told her not to do that because she needed that money for her school books but she didn't care of my words. when we came back home my parents got angry with me because they told it was my responsability, the punished my and they gave me a lot of slaps. what can I do to solve my problem? thanks