Is the questioner sinful for cutting off ties with her husband's brothers' wives, even though her husband urges her to visit them, knowing that her return to them would mean a return to backbiting and hypocrisy?

If the intention behind cutting ties is to avoid mixing without severing relations, then there is no harm. However, shunning (hajr) is not permissible for a Muslim to do to his brother for more than three (days), except for an overriding benefit. So, if you are harmed by associating with them or fear a deficiency in your religion, then it is permissible to shun them, and it might even be preferable. It is recommended to advise them with wisdom and good admonition, and to supplicate for their guidance and righteousness.