Just punched your best friend in the face? Put some ice on it and he'll be fine. Just wrecked his car? No prob, he's got insurance. Slept with his ex-wife/girlfriend? Whoah momma, you're in for a world of hurt!
Even among the closest Heterosexual Life Partners, there are some lines you just do not cross, and one of them's dating, sleeping with, or otherwise getting involved with a woman your bro [...] [tvtropes.org]