Is it considered a sin for a wife to be unable to forgive her mother-in-law who wronged her and slandered her, and to avoid her, especially with the husband not defending her?

"Spying on Muslims' private lives and harming them through slander and calumny are among the greatest sins. The default is that the wife's mother is innocent of what has been attributed to her unless proven otherwise, especially given her denial, coupled with the importance of having a good opinion of her. If it is proven, then the husband and her children must advise her and prevent her from injustice, for it is permissible to gently object to one's parents. It is permissible to shun someone whose evil is feared, but part of perfecting kindness to the husband is to be kind to his family and to forgive their slips. Forgiving the oppressor is better, and one must forget the matter through remembrance of Allah, seeking forgiveness, and reciting the Qur'an."