My mother because she is an attention-seeking bully. She liked to be physically assaultive when I was young, and enjoys indulging in daily temper tantrums to this day. Now that I am no longer available for her to yell at, lie to/about, manipulate and use, she yells at my father every time she sees him, which is at meals. She deliberately starts fights so she can yell at people - I think she feeds off the energy in some kind of parasitical way or something. She views the other people in the family as things for her to use for her own selfish purposes, not as other people with feelings, values and purposes that are important. Only she has feelings, etc.
I used to feel protective of my father because I stupidly believed he didn't know what was going on, but once it became clear that he did know and he just didn't feel protective enough of his children to stop her, I let him lie in the bed he had made. If he had bothered to stop her tantrums when he was younger and more spry, he wouldn't be having to put up with them now (if he had succeeded; since he never tried, I don't know for sure if he could have had any effect on her nasty, selfish behavior.)
I agree with Triangular Trees-it's how people treat each other that makes you family, not bloodlines.