What is the ruling of Islamic law regarding a second wife who stipulated to her husband that he should not reconcile with his divorced wife?

If the second wife stipulates to her husband that he must not take back his first wife, and that she will be divorced if he does, then this condition must be fulfilled as long as the first wife's 'iddah has ended or she was irrevocably divorced. The proof for this is the Prophet's (peace be upon him) saying: "The conditions most worthy of fulfillment are those by which you make intimate relations lawful." However, if the first wife is still in her 'iddah from a revocable divorce, then this condition is not permissible and invalid, because she is still considered a wife, and the Messenger of Allah (peace be upon him) said: "No woman should ask for the divorce of her sister to empty her vessel." And any condition that is not in the Book of Allah is null and void.