A Reddit user shared how his family unraveled after his brother left his wife to marry the woman he’d been having an affair with. Although the family tried to tolerate the new sister-in-law, everything shattered during a Sunday gathering when she told his wife she should take a paternity test because their daughter didn’t “look” like him. Shocked and insulted, he immediately defended his wife, saying she didn’t owe anyone proof.

When the SIL pushed again, implying he should “know the truth,” he snapped back and called her a mistress who shouldn’t judge others by her own standards. His mother scolded him for causing a scene, but he loudly reminded everyone that the woman questioning his wife’s fidelity had been the one sleeping with a married man. He and his wife left the dinner immediately.

Later, his brother demanded an apology, explaining he only cheated because his first marriage was arranged. The OP refused, saying that didn’t erase the affair or justify attacking his wife’s integrity. He also made it clear that he wouldn’t let anyone shame his family—especially not someone whose own actions had already broken theirs apart.

Online commenters overwhelmingly supported him, pointing out that cheaters often project their insecurities and that the SIL’s accusation was deeply disrespectful. Many noted that the real issue wasn’t the OP defending his wife, but the family enabling toxic behavior just to “keep the peace.”