Does a divorce conditioned upon the brother marrying the wife's sister take effect if the intention was only to threaten, and is it valid to condition a divorce upon the wife's mother?

If you said to your wife, "If this marriage takes place, then you are divorced," and you wrote that to her with the intention of divorce, or you threatened your wife's mother with divorcing her daughter if the marriage occurred, then you have made her divorce contingent upon the marriage taking place. Since the marriage has taken place, a single divorce becomes binding upon you, unless you intended multiple divorces. The intention of threatening does not prevent the occurrence of a contingent divorce according to the majority of scholars. You may reconcile with your wife before the expiration of her waiting period (iddah) if this was the first or second divorce. However, if it was the third, she becomes unlawful for you and is not permissible for you until she marries another husband. The waiting period ends with her purification from the third menstruation, or after three months for a woman who does not menstruate, or upon delivering her pregnancy for a pregnant woman. Reconciliation occurs through an explicit statement or an action such as intercourse. If her waiting period ends before reconciliation, then a new marriage contract must be concluded.