Maomao was putting two and two together. Specifically sniffing around the medical house, finding Rouran's insect book. Given her already proven detective abilities, she was well on her way: to 1) recognizing that the old maid running the clinic was actively involved in killing infants (by suppressing info about the face powder); and 2) recognizing that consort Rouran (daughter of She Shou) was tied to that clinic.
Secondly, it's possible that they intend to use Maomao as a hostage against Lakan. Alternatively, if they killed Maomao at that time and that was discovered, it would have brought the wrath and full attention of Lakan.
Thirdly, it's possible that Rouran personally did not want to kill Maomao (noting that she just saved Maomao from a whipping by lying). Perhaps even Sui Rei likes/respects Maomao.
Fourth, they may have wanted to find out what Maomao knew already and who she had told.
Fifth, they may have thought that Maomao had some utility (as apothecary, etc.) (Just bribe her by allowing her to be a mad scientist!)
Sixth, maybe they just like cats.
