I don't know what to do about my cousin?

I'm an 18 almost 19 year old boy.Going to school for cosmetology.About a month ago I took in my 17 year old cousin who was having trouble at home. Her grandmother was abusing her both mentally and physically. She was living with her grandmother because her parents did the same thing.So I helped her pack and i moved her in with me.Since she's been with me,she's had violent fits,temper tantrums,etc,over nothing. Just today she started screaming and hitting the wall and threatening to kill "Everyone and everything in this damn house." Because her ipad stopped working.She's been blaming her behavior on mental illness, but refuses to let me take her to a therapist. And I don't know what to do. I don't like leaving her alone for fear she'll hurt herself but I don't like being around her because I'm afraid she'll hurt me. And her suicidal and violent behaviors are messing with my depression and anxiety. She has nowhere else to go, but I can't keep dealing with this. What should I do?I'm so stressed out.
Added (1). Another thing,is just tonight she's started getting her boyfriend (who's 20) to start harassing me because I scolded her for how she was acting.
Added (2). I like the advice the first person gave me,minus the boyfriend part. She can't move in with him because he lives across the country. And honestly, they argue and fight every day. And also threaten to kill each other on a daily basis. I wouldn't want her living with him anyway.His parents (who he lives with) don't need that stress.

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