Conditional Divorce with the Intention of Prevention

He also asks and says: I am married and have children, and I am living in the best of circumstances. My brother wanted to marry my wife’s sister, who is divorced due to a lack of offspring, as he says. I refused and said to my wife’s family: If you marry my brother, my wife is divorced. My intention was sincere regarding the divorce if the marriage took place. But my brother married this woman. Does my wife become divorced? I do not want to divorce her, but I want to prevent them from this marriage. Please give us a ruling, may Allah reward you.
Answer: If your intention in doing that was to prevent him from marrying, and your intention was not to effect the divorce if he married, then you owe the expiation for an oath, and there is no divorce for your wife. If this is your intention and your intent—that you said this so that your brother would refrain—then this has the ruling of an oath, and you owe the expiation for an oath. This is feeding ten poor people, or clothing them, or freeing a slave. If you cannot do all of this, then you must fast for three days.
The presenter: May Allah reward you.