I suspect my cousin has mental issue. What to do?

I recently moved back home to be close to my parents. My parents have been living with my aunt and her daughter. For the past month, I have witnessed my cousin yelled, screamed, bashed things, cursed her mom and everyone else and even hit her. It was very traumatizing up to the point I hid myself in my room shaking. She has many issues in life, a broken marriage, a low pay job and low self-esteem. I cannot stand watching her abusing her own mother. I can't sleep at night because of her yelling and breaking things. It's my parents' house and I'm scared for them living in this condition. I can't tell her to move out because that will hurt my aunt and we care a lot about my aunt, she's over 70 yrs old. I have a good job and can easily live on my own, but I moved back cuz I'm worried about parents and my cousin. I think she has become crazy. I don't know what to do. I can't live like this..listening to her screaming all the time. My parents don't have the heart to kick her out since she can't afford living on her own. I'm scared the neighbor will call police on us. It's been such a disgrace. I really don't want my old parents to have to live with her and suffer like this. I know I have hit my limit and may even call the police on her if she kept abusing her mom and be violent like this at 11pm to midnight. I could not sleep every time she goes crazy. Sorry for the long post but I'm so scared and frustrated.