inb4 "They have to get married because the mc has a special bloodline ability" or something like that. And the family he's marrying into is one of the only ones compatible with that ability.
There's no way someone is this fucking stupid, come on author... At this point, he should be thinking about whether to accept her feelings or refuse her, not if she likes him as a man or brother, since it's very fucking clear she likes him as a man.