What is the ruling on helping one's father marry for the third time, knowing that he is financially dependent on his children and the mother rejects this, and is the refusal to help considered a sin?

The maintenance of a poor father is obligatory upon his children, without dispute. A condition for the obligation of maintenance is that the child finds what to spend in excess of his own needs. If there are affluent children, both males and females, then the maintenance is obligatory upon both of them. If the father is in need of marriage to preserve his chastity, then it is obligatory upon his affluent children to marry him off, even if the mother refuses, unless his chastity is preserved through their mother, in which case it is not obligatory for them to marry him off, but it is recommended. It is proper to combine honoring both parents and obeying them.