I dunno i think she mentioned something about perhaps ruining the mothers life before too? And this seems to be world that is hung up on these skills so perhaps disowning him as a child is actually the only way to make it a better life for him? Similar to how in other isekai type things the villainess' become commoners or something to gain a safer better life?
His mother is trying to give MC freedom and to not tied to the family responsibilities. Mother know MC true power, she know MC won’t be able to learn anything if he stay there. Mother want her son to pursue his dream of learning magic. But because MC is too OP, mother have no idea how she can help him, when the teachers she tried hard to find were all useless.
This family, because they are stationed by the King to protect the border, members of the family are not allowed to leave their territory if not by royal order. If they do so without royal order or an acceptable reasons, they will be marked as rebels and will be punished.
MC was announced as skill-less in front of many people. But the detail of the appraisal was not announced. That detail was taken by the mother, and mother do understood the true meaning of it.
Because MC was announced as skill-less, the King and people will not accept him as the heir.
Mother thinking of MC wish, she decided to make everyone to treat him as non-existence, this making people to not having their eyes on MC, as if he is disowned.
If he was never existed, then he is not a member of the family, so he is not bounded by that rules which tied him to that territory.
The reason why mother treated him coldly after that announcement and tell everyone include the sister to do so, was to make sure this plan won’t be discovered by anyone, not even the sister.
So basically, she take the part to play as the villain here just so her son will have the freedom to pursue his dream and so the Kingdom or anyone else will not spy(notice) him.
Because if it’s discovered MC true power, there will be a lot of people wishing to control him, or have his power, or aim for his life. And if they know that the family have good relationships with MC, they can take them as hostage, or as bargain to either control or attack MC. Mother wanted to avoid all those. She understand that her son is someone who shall not be limited to that small territory. Her son is fated to take on the world.
can i say fuck the mom because if that was my kid with no magic i would had left with him not push him away regardless of my status you should not leave your child because you will regret it later