What is the ruling on my oath upon the Quran to inform my husband's brother about my husband's marijuana smoking, given that there is no expiation to annul this oath except to inform him, and what should I do if I fear that problems will arise between the brothers if I inform him?

Swearing an oath on the Quran or in sacred places confirms the oath and makes it more binding and serious. A Muslim should respect their oath. If fulfilling the oath leads to a greater harm, one must break the oath and offer expiation by feeding ten poor people, or clothing them, or freeing a slave; or, if unable to do so, by fasting for three days, based on the Prophet, peace and blessings be upon him, who said: "Whoever swears an oath and then sees something better than it, let him do that which is better and offer expiation for his oath."

Accordingly, if informing your husband's brother about what his brother did would lead to greater problems or harms, then do not inform the brother. Advise your husband to cease the unlawful act, and offer expiation for your oath. However, if informing the brother would not result in harm and might lead to your husband abandoning the unlawful act, then you should fulfill your oath and inform the brother.