It is said that his father gave the family estate to Ujiyasu, who was his fifth son, instead of Ujiyasu's older brothers who had all reached adulthood, because the father thought the government would possibly not be apt to confiscate the family estate from the little child Ujisaki even if he was defeated to death in the battle.
五男である氏泰に家督を譲ったのは、当時成人していた四男までの息子より、幼少の氏泰に譲っておけば、仮に自身が敗死してもその後、幕府から咎められて取り潰される可能性は低いであろうと考えたためだと言われている。