After the Divorce, the Aloof Marquis Chased His Wife to the Crematorium

Three days ago, Su Qingmo fell into the lake "in a daze" and his body was not even recovered. The old lady of the Su family, Ma Liuer, made the decision to marry her sister to Gu Rinchuan, the Marquis of Zhenbei. Why? Isn't it just to keep the Su family's power?