In the first month after babies are born, they basically just eat, sleep, and cry while soiling their diapers. So, even if the children are sluggish and sleep more than normal, it can be difficult to detect.
The twins showed the same symptoms as Bai Ruozhu, being lethargic and excessively sleepy. Given that infants' vision is not fully developed, the poison affecting their sight wouldn't be easily detected either.
Indeed, the twins and Bai Ruozhu were poisoned with the same toxin, and the twins were poisoned first.
Bai Ruozhu finally realized why she was in such a hurry to assist Yu Hongxiu in giving birth and, because of the sudden discovery of Deng Shi and the midwife's conspiracy, she overlooked this. Or rather, she never imagined that Yu Hongxiu would get poisoned, especially a poison aimed at her unborn babies.