Is my boycott of my maternal aunt and her family—who prevent my mother from seeing my grandmother and mistreat us—considered a sin, especially since I feel humiliated when visiting them?

Maintaining kinship ties is obligatory, and severing them is forbidden. It is not permissible to cut off relatives due to their mistreatment, insult, or their cutting ties with you; rather, you must maintain ties with them. This is indicated by the Almighty’s saying: "Then would you, perhaps, if you turned away, cause corruption on earth and sever your [ties of] kinship? Those are the ones whom Allah has cursed, so He made them deaf and blinded their vision." And the Prophet, peace and blessings be upon him, said: "The one who maintains ties is not the one who reciprocates, but the one who maintains ties is he who, when his relatives cut him off, maintains ties with them." However, if visiting would cause harm, it is permissible to suffice with maintaining ties through means other than visiting, such as telephoning.