I feel like LW is projecting a lot of their own feelings on love and marriage onto their brother. While it probably would, indeed, crash and burn if everything they perceive is correct, I kind of feel like two 30-somethings know what theyre in for. Maybe one or both are demi or ace? maybe they wanna give a middle finger to the traditional nuclear family? Its absolutely OK to have one partner for love and one for companionship if everyone is cool with it.

ETA: also, aromantic people exist and they deserve to be happy too.