What is the living fuck is this manga manga my Man NTRed right in front of him and was way too fucking chill about that I’m in for this ride.
As ridiculous as it looks to us, i guess it makes a bit more sense through the lens of this ridiculous world's logic.
Haruka has already given him a child, and she's already reaching the (ridiculously young) age at which a woman is considered an old hag.
He says himself that they haven't really had sex since their child was born, so he's already looking at her as if she's an old grandma.
If you think of it like that, it makes more sense that he'd be happy to hand her off to his brother who still loves her like a young woman, so he himself can get a younger woman who he's still attracted to, as is the norm in this world.