I kinda wonder if he found out about her being ten that day, but I have a feeling he knew before since he knew where she lived. And that he went to her family for permission and kept calling her a minor. He never did see her in her proper child form though, and he was getting fucked up by the bug for a bit of that. So maybe I can see him not wanting to register the facts on paper and just falling for what looked like a woman that fell from the sky? I don't know, I'm honestly glad he's dead cuz that was the only outcome for that romance. It really was my main issue with this story. I have no love for the early 2000s or earlier manga being ok with children in relationships with adults. Makes me hate the brother in the story too, he definitely knew but still let his sister do that