Is the son considered sinful if he completes his marriage to the woman he chose despite his father's objection and slander against the wife's family? And what is the retribution for the groom's father for this calumny and injustice?

If a father slanders the deceased, he has committed a grave sin and must repent and seek forgiveness from those he wronged. Parents should not refuse their son's marriage to a woman of faith and good character without a valid Sharia (definition)-compliant reason. He should try to convince them. If they insist on refusing, the general principle is for him to obey his parents, unless he fears harm by not marrying. If marriage becomes easy, then good; otherwise, the absence of facilitation, as a result of Istikhara (definition) (guidance prayer), indicates that good lies in not marrying.