I find her kinda sus tbh. And there is a saying: "keep your enemies closer". So it's possible that she might not be thrusting her, rather keeping an eye on her?
There are some other stories of servants who worked for the family for years, only to be plotting behind their back while earning their thrust. To later on betray or take revenge when they think it's time.
The more people know the harder it is to keep it a secret. Not to mention she has clearly expressed her dislike for Miesa and has taken liberties that were not ordered out of her i don't think they would trust her to keep quiet if the family is in danger.
I kinda get the assistant’s (forgot what her role is) distress, what I don’t get is why they won’t tell her about Miesa. I mean, if she trust her and always with her, they should tell her.
Honestly, if my boss is making an insane woman the lady of the house - which may cause the house’s downfall or King’s wrath, Imma resign than be beheaded