Is a father sinful for not reconciling with his parents who wronged his siblings and mother?

Boycotting one's parents is forbidden and impermissible. The rights of parents are immense, and Allah has commanded kindness and good companionship towards them, even if they are disbelievers. The father must mend what has been broken between him and his parents, and he must treat them kindly and obey them in what they command him, such as mending his relationship with his sister. Whatever the case with the sister, she is a kin whom one must maintain ties with and whose severing of ties is forbidden. Maintaining ties means that one connects with his kin even if they cut him off, treats them kindly even if they mistreat him, and gives to them even if they withhold. However, if the abandonment of the sister is to deter her from her injustice and aggression, there is no harm in boycotting her if it is strongly believed that this will deter her. The grandparents also bear responsibility for their boycott of the father, for the child has rights over his father. The questioner can leverage his position with his grandparents to mend relations and bring viewpoints closer, for this is among the greatest acts of devotion and noblest good deeds.