The poison was a bit tricky --- it wasn't that dangerous as long as it was treated by her acupuncture at the right time. However, if it was already passed the golden time, then there was also nothing that she could do, except prolonging his life.
Yao Ling made this poison by learning from a rare book that she saw at Wang Luo Hai's study room. Yao Ying brought it back to her because of her request --- of course, with Wang Luo Hai's permission. Thus, she was able to learn various types of poison. It was important for her, after all, she needed more knowledge to understand Mei Li Gu Niang's poison.
For the last reason, Wang Luo Hai was oblivious about it. They didn't dare to tell him Yao Ying's abnormality until they were sure the reason why Yao Ying wasn't affected by it.