Mrs. Jing didn't care about Jing Yuan's behavior. She was only focused on fulfilling her mission. It's not that she had feelings for Jing Xiaonan, but rather that she had a task to complete. Once it was done, she could leave. The thought of her son acknowledging another man as his father for more than twenty years had made her nauseous.
If the man wasn't the one she loved, she wouldn't have gone to such extremes. Knowing he would never love her as much as she loved him, she still adored him devotedly, even deigning to lay under a man she didn't love. Only she could understand the agonizing feeling this brought. Seeing Jing Xiaonan's daughter acting this way before her marriage, Mrs. Jing felt a perverse sense of pleasure.
It's hard not to say that the hearts of the Jing family members had grown cold. Each one exhibiting signs of psychological distortion, only Jing Xin remained sane, though he no longer seemed to reside here.