Is the complainant considered sinful for backbiting her maternal uncle? Will her repentance be accepted? How can she rid herself of this sin? Does the lack of repentance from the maternal uncle and maternal aunt (the complainant's mother) mean that Allah will not accept their deeds or forgive them?

May Allah reward your efforts in reconciling differences. Regarding backbiting, if you backbit your maternal uncle with things that he dislikes and are not true, you must sincerely repent, feel remorse, not repeat it, and seek forgiveness [from Allah]. If you can seek his forgiveness without causing harm or scandal, then do so. Otherwise, suffice with seeking forgiveness [from Allah] and mentioning his good qualities.

As for your mother and your uncle, it is not permissible for them to abandon or sever ties with each other. Abandonment among Muslims is gravely forbidden, and it is even more severe among relatives. They must both seek forgiveness [from Allah], reconcile, and overlook each other's mistakes.