How should one act regarding the claim of the husband's relatives that they harness the spirits of righteous saints to consult them and help them draw closer to God, and what are the consequent marital problems that arose from rejecting this?

Seeking assistance from the spirits of righteous saints to fulfill needs is a form of charlatanism, sorcery, and the domination of devils. There may also be sorcerers behind it, whom devils do not serve except after disbelieving in Allah, as stated in His glorious words: "And they followed what the devils recited during the reign of Solomon. It was not Solomon who disbelieved, but the devils disbelieved, teaching people magic and that which was revealed to the two angels at Babylon, Harut and Marut. But neither of them taught anyone until they had said, 'We are only a trial, so do not disbelieve.' And yet they learn from them that by which they cause separation between a man and his wife. But they cannot harm anyone thereby except by permission of Allah. And they learn that which harms them and does not benefit them. But they had already known that whoever bought it would have no share in the Hereafter. And wretched is that for which they sold themselves, if they only knew." These people should be advised and reminded of Allah Almighty, preferably by scholars. As for marital problems, no one should be definitively accused or blamed without clear evidence. If there is a strong suspicion of magic, those affected should resort to lawful incantations (Ruqyah Shariah).