Why didn't Grandpa Su let Tucki marry Jixiang?

Grandpa thinks they are cousins and can't get married. Later, he learned that Tucki was adopted by his parents, because Tucki's adoptive father could not have children, and his grandfather forced them to have children. In desperation, the couple adopted Tucki. Later, on his 25th birthday, his biological father transferred all his property to Dao through a lawyer, only to realize that he was not his own grandson.