Aside from the possibility of her mother trying to keep Miyo safe from something or other, it's far more likely that Miyo's powers being sealed or semi-dormant has to do with the Usuba family itself or the nature of what ever her power is. The timing obviously has to do with getting treated like a human being rather than a slave, though. From what of the novels has been translated thus far, this seems to be even more likely (but nothing has been confirmed). The only thing that has been confirmed is that her mother ran away because she was basically going to be used as a breeding mare, since the children of people with her own ability have the strongest ability of those held by the Usuba family. Whether this is actually the case for Miyo… hasn't been translated yet.