According to the Prince's Gensopedia entry which was translated by the Suikoden Kiwami Encyclopedia. It turned out there was a time when first born males are killed in order to prevent bad luck and disaster from affecting the nation. That practice had to have been abolished long before the events of the game since it was never brought up once. Must have been overturned by a Queen that had her baby boy taken from her and murdered.
It's in the first paragraph here.