Jin is 50% shark/50% mer, and Kaori/kenjaku a regular mer. She was most likely drawn to Jin because of his unusual heritage.

Regarding Yuuji’s family situation, I've been thinking that Kaori/Ken has had some unusual children before. Like, Choso is off somewhere, being an orca/mer mix or something, haha. Yuuji probably doesn't know he exists 🥲

I haven't decided if Kaori is around or not. She might've taken off to find more interesting partners, or she's just not as worried about Yuuji and therefor not as involved as Jin is. She'd probably love that her baby was mated to a real shark, though😆

Sukuna might've eaten the mate. Whoever they were pissed him off too many times for him to put up with them anymore. Needless to say, he didn't care about them even to begin with. He just figured it was time for a mate and later regretted his choice. And now he dove head-first into a new relationship so he learned literally nothing LMFAO

I imagine the shoal being nomads, so they don't have houses, but otherwise they're pretty advanced. They have doctors, inventors, scholars, etc, so they've figured out how to make glasses at some point. They just stick to inventing small things since they move around a lot.

The only similarity Yuuji sees between Sukuna, Jin, and himself are the colors that he notices when they first meet. He just thinks it's a common shark color, but it really isn't. The thought that they could be related definitely doesn’t strike him. Sukuna thinks Jin has been dead for well over twenty years so he doesn't think Yuuji can have any relation either (not that I think he'd care since he's the type to take what he wants regardless of the consequences).

I hope that answered some of the questions 🩷 Thank you for asking 😊🩷🩷🩷