Is the fiancé sinful for preventing his fiancée from confronting his mother about the slanders she attributes to her, and must the fiancée endure the mother-in-law's injustice?

If the fiancée is still a stranger to the fiancé, their interaction should be based on this until the legitimate marriage contract is concluded. If the fiancé's mother accuses the fiancée of false accusations, the fiancée has the right to defend herself and refute the accusation by speaking frankly, and the fiancé does not have the right to prevent her from doing so. The fiancé should defend his fiancée and advise his mother, because supporting her means defending her and preventing injustice from befalling her. It is advisable to act with wisdom and deliberation and try to win the mother's affection to avoid problems that might affect married life.