I had assumed the incest thing was just his own family being messed up, but no, if this is anything to go by, it's a normal and accepted thing in this world.
I'm, like, 97% sure that the debt itself was faked and the brother is just being a wingman in a very questionable method. I mean, it's already been established that the guy could already pay back the 500 yen with ease, but he still supposedly can't. In other words, I bet he has no intentions of...
Before even reading, here's my guess: this was the actual reason for the lending money, the dude was being a wingman for his sister, and has no intentions of paying back. Very questionable methods, though.
I'm calling it now, she's arranging a meeting between the three of them. This isnt a drama manga, and I don't see any prolonged misunderstandings happening, so chances are his wife will show up there too.
I'm just gonna pretend that his Half Vampire thing also makes him immortal so that he won't die and leave her alone. That's the only way they get a permanent happy ending even without him becoming a full vampire.
So she raises a good question. Why DOESN'T he say anything? Is the secret plot twist that they are actually dating and this is some kind of play they got going on mutually consensual during school hours?
And I still can't tell if she likes him, or if she is just horny and likes his reactions. Are we dealing with a girl who is teasing the guy she likes, or just a bully and a sexual deviant?