Does this make the 3 of them step siblings since his mom was with their father

Ayy yoo i never thought of it like this but I don't think so. Because he is a pure breed pufferfish meaning the mom was forced to copulate with another pufferfish just like how the mc was forced to breed with another pufferfish.

I think you were thinking about half siblings, and op is kinda right that they're step siblings. Though MC's mothee and the twin's father probably weren't married so idk how this works