In 1242 when his father died, he came into conflict with his younger brother Yasutsuna SASAKI who had handed over a fief and succeeded the family property from their father during their father's lifetime; in 1243, he instituted a suit against the bakufu over this matter, and Shigetsuna was granted a fief owned by Yasutsuna in Omi Province by the bakufu.
しかし仁治3年（1242年）に父が死去すると、父の生前に家督と所領を譲られていた弟の佐々木泰綱と抗争し、寛元元年（1243年）には幕府に対して訴訟を起こし、幕府により泰綱の近江国内における所領を譲り受けている。