To what extent is a wife obliged to obey her mother, who insists that she go to a healer who claims to use jinn to treat the illness of "Umm al-Sibyan"? Is the refusal of the spouses to do so correct, and what is the best way to seek treatment?

What the questioner and his wife are doing by not going to the woman mentioned is correct, as it poses a danger to their faith. The limits of a wife's obedience to her mother are in what is good and what does not involve disobedience. Obedience is in what is right, and there is no obedience to a creature in disobedience to the Creator. The best methods of treating the obsessive thoughts and psychological illness that the wife is suffering from are the Noble Quran, legitimate ruqyah, and prophetic supplications.