Merci...

I want to tell my sister that a large part of why her relationship is failing is her fault, but her response will be screaming, cursing, rolling her eyes and a variety of other means of expressing that she's ignoring me and believe everything anyone else says is wrong because she knows all.
Her relationship is a series of arguments and ultimatums initiated by her, usually. Her boyfriend is not the type to start an argument. He definitely instigates when he should be more mature, but he's a 21 year old kid. He likes getting a rise out of people. He likes picking on his girlfriend and playing too much. He's at a point in his life where he's at his least intelligent and most reckless and irresponsible.
He only ever complains about her when she goes bat-shit and starts screaming at him, calling him a fucking idiot, a retarded piece of shit and other derogatory things that would have been the end of a relationship for me.
When she gets drunk she's a raging bitch to everyone and I find her practically impossible to tolerate, yet he still manages to tolerate her twice as long as the last person to drop out of the race.
He takes all of her shit and rolls with it better than most sane people would, but she seems entirely incapable of offering him the same. Everything he does makes her angry. They live together, he was working two jobs and they didn't get to hang out much but when he has free time he wants to shoot the breeze with his friends and play golf or Madden or what have you. She goes nuts because she feels he should be spending all of his free time with her, but she's attacking him for that free time on his first solid day off.
I understand it from her perspective, I really do. I do believe they should spend a little more time together. However were I in his position, I wouldn't -want- to spend much time with her because she turns into an insufferable bitch when things aren't going exactly as she wants. If he answers the phone when they're doing something or makes a joke at her expense or doesn't find a subject she's discussing 100% compelling she immediately goes off on him and starts calling him names or complaining about beef from weeks in the past.
One thing you never do is bring old shit to a new argument, but try telling her that.
She's impossible to explain anything to and never wants to see things from anyone's point of view but her own. She's totally unreasonable, so how do you reason with her and make her see that if the relationship fails, its her fault as much as anyone else's?
I don't want them to break up because they're two stupid kids. If they break up it ought to be for a legit reason, but what the hell does someone say to a kid that throws her cellphone when she's pissed, of a kid that laughs everything off and never takes anything half as seriously as they should?
Geez-oh-petes.