Here We Go Again

my sister and her boyfriend are fighting again. they fight over the stupidest things and instead of talking it out like rational reasonable adults they scream and holler at each other. invariably it gets to the point where he starts throwing things.  i've told her that if he does it again i will call the police and have him taken out of here because there's no way i'm going to stand by and let him hurt her in the middle of his tantrums. he starts screaming and it's the same thing over and over. tonight i stepped in and yelled back. told him she'd answered his question, he might not like the answer but she answered it so to stop screaming and repeating himself.  he thinks if he gets louder and repeats it she's going to change what she has to say. they've been together long enough now for him to know that she's set stubborn once she's decided something. getting louder does nothing and she's certainly not intimidated by him so who's he trying to prove himself to? i feel badly. i think my sister was wrong to say 'shit happens' when he started in about his grandmother who's funeral was last week. that was cruel and an empty gesture meant to hurt him. she lashes out verbally, he lashes out physically. he's never hit her but i don't like his lack of self-control or his inability to view himself in the wrong. no one is 100% right all the time but he never acknowledges his own failings then gets angry and hurt when she points this out.  it's just a mess all around and i'll be glad when january gets here. she's given him til then to get counselling for his issues and get his act together. she's not one to suffer a fool. and he's so immature in so many ways. so coddled by his mother to make up for her leaving him to be raised by his live-in grandmother when he was young. but indulging him has only made him feel entitled to respect he hasn't earned. and when confronted he's so certain everyone is against him (because of course he's never in the wrong).  it's tiring trying to mediate between them.