What is the ruling on one who severs ties with a relative (his sister), knowing that his sister has severed ties with their sick mother for three years?

The status of kinship ties (رحم) is great in Islam, and severing them is forbidden. If the kinship tie that is severed is that of the mother, the sin is even greater. The daughter who has cut off her mother should be advised and reminded of Allah's punishment. It is not permissible for the brother to cut off his sister; rather, he must continue to advise her and supplicate for her, unless shunning her would deter her from severing ties with her mother.