I have watched the first and second seasons of Xia Na animation and the novel 0 ~ 2 1 n times, and I can tell you for sure.

Tomoko's father, Kotaro Sakai, and his mother, Chikusa, are both ordinary people. They got married when they were students. Youer is their second son. Youer had a brother who died unfortunately after birth. The reason why it was named Youer was to prove the existence of Youer's brother.