Should I attempt to fix my seemingly broken relationship with my sister?

My sister and I have always been close. She is currently 17 and I am 19. We have both been involved in the arts since we were young, and recently she gave up ballet because she couldn't handle the pressures. I continued with music and ended up getting into a university overseas. Before I left she was really happy for me and we were excited together. She took the year off from school to recuperate after leaving ballet. While I was away my parents said she was becoming more and more difficult and saying some really hurtful things. I encouraged them to keep being gentle with her as I believed it would help. My elder sister moved back home, and yet my younger sister (Amy) was so horrible to her, my older sister had to move back out. When I returned home for the holdiays, my sister was like almost a stranger. She refuses to speak to me, and tells me that I think I control her and I treat her like a child. I don't know what to do!!!! I feel so helpless because I am so angry that she treats my parents so badly. yet she doesn't listen to a thing I say. I still care about her, however she doesn't seem to value that at all. Should I leave her alone? Or attempt to fix it? ANy advice would help please! ! ! !