How can we resolve this? Should I get involved?

Okay. My husband hates his sister because she's a spoiled brat. He's 23, she's 19. He also has an older brother (26) who treats everyone like crap (makes more money than anyone but got NO ONE a christmas present)... well, anyway... My husband got into a tiff with his sister (they're two adults mind you) and his parents are trying to get right in the middle of it. Well, they took her side without even asking for his story. His dad called him a Heartless Son of a B**** because he told his sister "thanks for making me stay 20 more minutes"... boo hoo. She went and cried to daddy. Well, now his mom keeps nagging him and nagging him and all he says is he doesn't want to talk about it. That they don't need to get involved. That he's an adult now. WELL, his mom has always thought he has a temper (HE DOESN'T... believe me, I know what a temper is-- i.e. alcoholic ex boyfriend.) and he's getting tired of hearing it. Well, the other night she tells him if he doesn't straighten up...