Because Yoshihisa had no sons and Tadatsune's elder brothers Tsurujumaru and Hisayasu died young, he married Yoshihisa's third daughter, Kameju, and became a successor as the head of the Shimazu clan.
義久に男子無く、また、忠恒の兄（鶴寿丸・島津久保）が若くして死去したため、義久の三女である亀寿と結婚し、島津氏宗家の後継者となった。