In 1859, when Sadamichi MATSUDAIRA, Lord of Kuwana Domain, died, his eldest son, Mannosuke (later the Lord of Kuwana Domain, Sadanori MATSUDAIRA), was only 3 years old; and Sadaaki, aged 14 at the time, married into the MATSUDAIRA family, as the husband to their then three-year-old daughter, Hatsuhime.
安政6年（1859年）に桑名藩主・松平定猷が死ぬと、長男・万之助（後の桑名藩主・松平定教）が3歳と幼少であったため、14歳で定猷の娘・初姫（当時3歳）の婿養子として迎えられた。