Complex sentence: Hojo Tokimasa helped Minamoto no Yoritomo, a son-in-law and after the death of Yoritomo, became a regent, gaining a real power in the Shogunate.
Split into simpler sentences: Hojo Tokimasa helped Minamoto no Yoritomo, a son-in-law. After the death of Yoritomo, he became a regent, gaining a real power in the Shogunate.