38 and 19 isn't *inherently* going to go badly - but that's predicated on the 38-year-old being mature enough for a relationship with a 19-year-old, and it sounds like your brother very clearly isn't.
So, yeah, what you can do is as the advice said, side with the girlfriend against him. Don't necessarily be confrontational enough about it that he cuts off contact about it or anything, but make an effort to side with the girlfriend against him even when she doesn't, make sure she knows you like her better than him, so that if she ever needs help against him you're already there, and that he knows you're not going to reinforce his bad behavior.
But honestly - a lot of 19-year-olds have bad relationships, it's a learning experience. Your brother being an asshole sucks for everybody who has to be around him, but it doesn't necessarily means she needs you to rescue her. Hopefully she'll figure it out soon, dump him, and find somebody who's at least as mature as she is to be with.
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So, yeah, what you can do is as the advice said, side with the girlfriend against him. Don't necessarily be confrontational enough about it that he cuts off contact about it or anything, but make an effort to side with the girlfriend against him even when she doesn't, make sure she knows you like her better than him, so that if she ever needs help against him you're already there, and that he knows you're not going to reinforce his bad behavior.
But honestly - a lot of 19-year-olds have bad relationships, it's a learning experience. Your brother being an asshole sucks for everybody who has to be around him, but it doesn't necessarily means she needs you to rescue her. Hopefully she'll figure it out soon, dump him, and find somebody who's at least as mature as she is to be with.