i don't think we can even say that it's a naive choice that the maid chose to follow the assholes, at the contrary, it's a pragmatic one, she know very well how a single mother is welcoming in this world, the raped maid say herself that they will be called whore and receive stone in the face in her village, and i don't think it's different elsewhere.
she doesn't know enought of astina to trust her, and from her point of view, if she say something or astina discover what happen between her and the baron, she can be kicked out of the mansion with nothing, at least with the baron, she know where she go because it's the continuity, between two shitty the fate, she chose the one who change the least.