Sounds to me like your brother is in an abusive relationship. That does not just happen to immature people you know. Be careful that you dont overlook the fact that your brother may be emotionally abused by a controlling partner, and that regardless of his levels of maturity, he can be a victim.

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Maybe your parents can intervene, maybe someone could reach out and tell him he has a safe place to stay if he wants to break away from her. But regardless often times people dont leave abusive partners as soon as they should. And there is a good chance, since he is a man, that people just dont even entertain the notion that he can be just as much under the thumb of an abusive woman, as any woman could, under an abusive man.