What should a man do whose brother's widow he married to raise her children, and now his wife is refusing this marriage and demanding a divorce or to have him to herself?

The Shari'ah has forbidden a wife from seeking divorce from her husband without a valid reason, based on the saying of the Messenger of Allah, peace and blessings be upon him: "Any woman who asks her husband for a divorce without any harm (to her) will be forbidden the scent of Paradise." Your wife's request for divorce due to your unwillingness to divorce your other wife is not a legitimate reason according to Shari'ah, and she is not permitted to do so, provided you have furnished her with an independent dwelling and fulfilled her rights. Advise her and remind her of Allah. If she does not respond, then involve righteous people to reconcile between you. If she persists, then abandon her to her family's house, and she will be considered disobedient (nashiz). If she does not return to what is right after a period, then there is no good in keeping her under your marital bond. We do not advise you to divorce your brother's wife due to the absence of a legitimate Shari'ah justification, and for the benefit involved in raising and caring for your brother's children, and for the great reward therein; for the Prophet, peace and blessings be upon him, said: "I and the guarantor of an orphan will be in Paradise like this," (and he gestured with his index and middle fingers).