I understand this argument but you have to take this scene into context. The MC was pursuing the sister constantly before this, and suddenly he comes down on her like a hammer. He implies that his sister is not smart enough and has no agency to make any choices without him. He then proceeds to give her guidance on sex, smoking, drinking, money, and relationships. If the MC was doing this to manipulate her then fine, but his thoughts seem to betray a feeling of undeserved moral superiority and altruism that I find laughable.