My theory is, her father was killed by her uncle and her mother was an alliance. They already had a relationship in secret even if her mother was already married. And her little brother is her uncles son.
GUYS HELP... MY PITY IN MU QIU CAME BACK WGEN SHE SAID SHE REGRETTED EVERYTHING. but i know once her heart will have another complications she still might think of killing either of the Song sisters