So, in the trial we learned that Asterion's father did have another son, meaning Asterion has another brother. Now, I know it doesn't make sense (for many reasons), but also during the beach scene, the fishes also came to surround Oscar as they did for Asterion. I understand this is most likely just a coincidence, after all, if I did the research, which I plan to later, we could probably find who the son was. Edit: Soo that was a bust lol.

Fish swimming to someone isn't necessarily unusual, especially in an area where they'd be used to people like a beach, though it's still possible they're (very distantly) related. Personally I'm leaning more towards the idea P mentioned though