Liu Li merely told them about how she wanted to kill Yao Ying and Yao Ling but her plan had failed and everyone had found out that it was all Madam Wang's plan. Yao Ling had told her not to tell anything else --- especially about Wang Luo Hai's infertility.
She didn't know whether Madam Wang's younger sister knew about it or not... Yao Ling was afraid that if this matter was known to her, the younger sister would back off. She was just doing prevention.
There were only three people who were there when Liu Li told them the story --- Madam Wang's father, legal mother which was also Madam Wang's birth mother, and the younger sister.