I'm so sorry this is happening to your family, I've known a lot of women, and I've seen women get pretty angry. That said, I think your intuition is correct. There is no excuse, in my opinion, for beating up your niece and standing outside her room and saying really mean things. I'm not a therapist or mental health counselor but, I'd be very considered that this person might have a personality disorder.I know a family very well where the son's spouse was the reason he cut ties with his siblings and his parents. While this was sad to see, a lot of manipulation of family members was taking place. Sometimes it is for the best. Perhaps he will find out, see the error of his ways, and come back. We can hope, but in the meantime the family home will once again be a safe and healthy place to be.
mrs