What must a father do to atone for his sin regarding what he did to his paternal aunt, and is the aunt held accountable for her actions and her invoking against her son, and to what extent is her sin in that?

What your father and uncles did—corrupting their sister's relationship with her husband, separating her from her son, and resorting to magic—is a grave sin and a major crime.

1. Corrupting the marital relationship: The Prophet, peace and blessings be upon him, said: "He is not of us who incites a woman against her husband." This is an act of sorcerers and devils, and whoever incites a woman against her husband is cursed. 2. Separating a mother from her child: This indicates a hardened heart and the wretchedness of the doer; motherhood is an innate disposition. The Messenger of Allah, peace and blessings be upon him, warned that whoever separates a mother from her child, Allah will separate him from his loved ones on the Day of Resurrection. 3. Resorting to magic: The magic of separation (sihr al-sarf) to separate the sister from her child, and magic is one of the gravest sins and destructive acts, as it necessitates polytheism with Allah, involves the use of devils, claims knowledge of the unseen, and involves forbidden actions.

Your father and uncles must repent to Allah and rectify what they have corrupted. They should treat their sister with ruqyah (definition) shar'iyyah (Islamic exorcism) and destroy anything suspected of being the cause of the magic. As for your aunt, she is not to be held accountable for what she did while under the influence of magic, as long as her will was taken away.