So the mom had a highly marketable skill, but instead chose to work as a maid. Then even after she had a daughter (and given the dynamics involved, the mother probably didn't sleep with the father willingly), she continued to stay while she and her daughter were abused. Why not run away? It doesn't really feel like the noble family would have wanted to drag them back.

Also the Duke and his servants come across as the worst detectives in the world. "Hmmm, why is this noble girl covered in injuries, always dirty, starved, and doesn't seem familiar with standard noble things. Could this mean something deeper? Maybe she's just weird, nothing else comes to mind that could explain it all."