Im questioning my sister's mentally stability please help?

Well she's starting to disturb me now. Her mood swings are constant and unpredictable. She's hostile and mean and she's weird. She says that she hates people, she stays in her room all day, and she recently told me she had a friend that was always with her but I couldn't see. She has cut herself in the past and talked to counselors at her school, but she's still the same if not worse. I was wondering if her aversion to people was do to the fact that when she was younger me and my older sister teased her a lot. We would gang up on her and talk about her flaws, and one was that she was chubby. Last year she actually stopped eating and lost a lot of weight until she actually started to look unhealthy. The only other traumatizing thing I could think of that might have contributed to the way she is now is the constant arguing between my mother and father. They would fight, yell at each other, break up, sleep in different rooms, and then be back together two days later. My mom used to pull out weapons and threat to kill my father so I think it confused her a little on how people are supposed to relate to each other. She writes stories and I came across some that were very violent and about killing my father and mother. She's also only 15 but I think something is mentally wrong with her. Please help me. What should I do, Do you think there's something wrong, and why do you think she turned out like that?