Miyo's mum is definitely sus. number 1, no doubt, plain and simple. In the earlier chapters, back in Miyo's childhood, her mum said something along the lines 'please forgive me for being selfish'. That right there is saying a lot.
I assume the main Usuba household given there withdrawal from the public almost seemed liked an enslavement lifestyle from within/outside and Miyo's mum thought by blocking her daughter's ability will help break her from that cycle of being traded and sought as a commodity bride amongst households, hence resulting in Miyo living a carefree life.
Now.. the seal is temporary weakening. I find very unlikely another ability user managed to bypass Kudou's front line defense, which means Miyo was subconsciously self harming/testing herself.