Is not wanting to see or greet one's maternal uncle considered forbidden (haram), due to his severe negligence in caring for his parents (the questioner's maternal grandparents), who have passed away, despite her and her family's care for them?

What you did for your grandparents is among the best deeds and acts of devotion, and it is a form of righteousness and kindness that Allah Almighty commanded in His saying: "And your Lord has decreed that you worship none but Him. And to parents, do good." As for your uncle's shortcomings regarding his parents, it is an act of disobedience and a major sin, based on the Prophet's (peace be upon him) saying: "Shall I not inform you of the greatest of major sins?" We said, "Yes, O Messenger of Allah." He said, "Associating partners with Allah, and disobeying one's parents." Nevertheless, it is not permissible to cut ties of kinship with him; rather, he should be advised to repent and be treated with kindness.