Mo Tiange considered a couple things before disclosing her secret Pure Yin Constitution.
First, Bixuan Court was a cultivation group for female cultivators. Among the six people in this pagoda, only Tang Shen was male and based on his actions so far, it was very unlikely he could do anything damaging to her. If Elder Qingxi was here, maybe she'd try to do something to her for Tang Shen's sake. Elder Qingyi and Elder Qingmiao, however, had no reason to force her.
Second, the six of them were trapped inside this Dao-Achieving Pagoda together, so now they were all in the same boat. There was no need for Elder Qingyi or Elder Qingmiao to harm her.
Third, although cultivators in the cultivation world were self-centered, cultivators from cultivation groups were extremely respectful towards the founder of their groups. Out of consideration for Mo Yaoqing, they wouldn't do anything to her.