In our Christian Societies, polygamy is rare... very rare.
It does happen, but not that much.

I don't know for Asia. And I don't know if it's in the mood of this anime.

As others, I feel that this 180° turn from Schinichiro's mother hides something.

I just got an idea... The father says it's impossible Schinichiro and Hiromi are siblings.
However, he doesn't say he isn't Hiromi's father.
I know, that last one is a bit stretched...