Find out more later but she used to know how to take advantage of trust and wreck the lives of the other concubines while getting away with it. She's an adherent in belief that what's good for her is good for Feng clan, and only she deserves to stand on top just because. Part of reason why so few kids is she'd send food over to the others under guise of being a concerned fellow wife wanting to provide healthy food for the baby when reality is that it'd be poison that'd cause a miscarriage. Likely didn't poison the mothers too was had a good grasp that Feng wouldn't look too deeply at something that couldn't be undone with a plausible excuse along with continued contributions from her father's family. Yet he would be enraged and feel would lose face if it became known that one of his wives was murdered and the killer left unpunished. Basically before resting on her laurels she was only good at pulling the other wives down and looking good to her husband with her family's help. As for why Feng acts so shameless and self-centered is in part due to his mother's bad teachings.