Partner in what? She's not his girlfriend & socially & financially he's above her & she does behave like an ignorant child. Him wanting to fuck her doesn't mean they're partners or on the same lvl.Its more a critique of his smugness not his actions. If your partner treated you as an ignorant child you would not be happy. Partners help each other through things all the time but it is definitely a different dynamic. This is regardless of whether the advice is right or wrong, needed or not needed. His concern on the beach in the morning comes across much better and is more understandable, than his condescending inner monologue the night before.
And I want to point out that this is the guy that gave his possibly alcoholic mother drinks on the beach to try to take advantage of her.
Can't really comment on the mother cause I avoid her route, he might behave like an asshole towards her, I don't know & I don't care, but that's not the point of the conversation, the discussion is about how he treats his sister. The way mc treats the mother has nothing to do with how he treats his sister.