Sakana is a mainlander in his 30s, it's likely he doesn't have a sibling.
(pretty sure he's a mainlander)
Dizzy has implied a few times that her parents, mom at least, immigrated. Her half-sister is the first born, with her brother being the middle child.
Also, in the 80s (relevant time frame), China had the "one-and-a-half-child policy" that could apply in certain cases in "rural" areas if the first born was a girl.
I'm not an expert in Chinese family planning, but I don't think the fish being a born mainlander is a terminal condition for the rrat.