Is a wife obligated to love her husband's mother, and is she sinful if she cannot love her, or if she informs her husband of his mother's troubling behavior that has caused disagreements between them, for fear of being among those who separate a son from his mother?

There is no sin in hating the mother-in-law due to her bad behavior, as love and hatred are matters of the heart for which one is not held accountable. It is wrong for the mother-in-law to spy or mistreat, and honor and mutual advice should prevail among relatives. There is no sin in informing the husband of his mother's bad behavior, but it is better to be patient and generally avoid informing him to prevent discord or the husband falling into disobedience to his mother.