Yokosuka Domain
Tadasaka was the second son of Sakai Tadazane, daimyō of Himeji Domain, and was adopted by the heirless 6th daimyō of Yokosuka Domain, Nishio Tadakata, to be his successor. When Tadakata retired from public life in 1843, Tadasaka became daimyō and head of the Nishio clan.