Is what was mentioned regarding the grandmother's and maternal aunt's refusal to be contacted, their harming of the mother and siblings, and the threat to report him, considered a legitimate justification for severing ties with them, and what is the minimum level of maintaining kinship ties that would not make a person a severer of kinship ties?

It is not permissible for them to harm your mother and family or to sever ties with you unjustly. It is incumbent upon all of you to end the estrangement.

If you fulfill your duty on your part and respond to a severance with connection, you will not be considered one who severs kinship ties. The sin lies with whoever refuses connection and insists on severance, for the Prophet, peace and blessings be upon him, forbade estrangement for more than three (days).

If they persist in their insistence and you fear harm from them, then you have the right to cease contact with them.