It was not a mistake, but a cruel scheme to exchange children. This wicked old woman stole the Xiao family's daughter and gave her son to them instead. The Xiao family had helped their enemy raise a child for more than forty years. Not only that, but she even treated the real daughter of the Xiao family very badly! In order to keep her from finding out the truth, she made her daughter bleed to death during birth.
If his father found out about this terrible thing, knowing him, things would not be good.
It would be better for him to leave the family now and cut off all relations with them from then on. If he was the one who had to find out that his biological daughter had been killed, he would have done much more than just kick his fake son out of the family.