I worry that maybe lady Han poisoned herself or the 4th miss did. First this would help draw up sympathy and attention for her and away from the new wife. Second, it can be used as an opportunity to frame and bring down one of their opponents in the household. Mind you, since they've been using the baby as leverage it would seem foolish to risk killing it. However, if they somehow know the baby is a girl or that the baby's appearance will likely reveal her cheating, they may have decided to capitalize on it the best way they could.