How can one atone for the mistakes of a deceased father towards his maternal half-sister, knowing that she refuses to forgive despite repeated attempts?

You must seek forgiveness for your parents during their lives and after their deaths. Increase your supplications for your father, asking for mercy and forgiveness for him, and try to seek out those he wronged so they may forgive him. For the rights and grievances of people must be fulfilled or relinquished by their owners, because if a person has wronged his brother, he should seek his forgiveness today before there are no dinars and no dirhams. And know that your seeking forgiveness for him and your endeavor to absolve him of those grievances is among the greatest acts of piety towards him after his death.