@Guroga probably because she doesn't actually consider him a threat but more of a foolish tool. She does punish him in small ways here and there and she'll probably give him due punishment at a later point in time when her own power is more firmly established and she has no more use for him. Right now she has yet to enter the prince's household, so she is still tied to the Feng family so it wouldn't make sense for her to destroy the head of the household yet when it could impact her own position. It suits her for now to just manipulate him and set things up so he becomes dependent upon her.