Is what happened considered a قطع للرحم (severing of kinship ties) on the part of the sisters who defended their mother, and were then prevented from visiting her except at their brother's house, with the mother refusing to meet them anywhere else?

The right of the mother is great, and honoring her, revering her, and obeying her is obligatory. If the sister has wronged her mother, she has committed a grave evil, and she must repent and apologize. As for visiting the mother, it is obligatory to visit her at the brother's house. However, if the brother expels [her] or prevents [her] from entering, then there is no blame in refraining from visiting her at his house, while maintaining ties with her through all other means. It is advisable to strive for reconciliation among family members.