All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is not permissible for you to leave your wife if leaving her will lead to her loss and the loss of your baby. The fact that your family wants you to leave her and divorce her is not a sound reason to desert her. You are free and you are responsible before Allaah and before His Slaves.
Moreover, you should know that it’s not of noble morals of a Muslim to be ingrateful to a person who did him a favour in time of need. A magnanimous man or a generous, free man never, never acts in this way.
So, how can a person do this with his wife and his son. Your wife gave up everything in this world for your sake and she bacame a captive in a foreign land. It is she who rescued you from the grievous murk and led you along the straight path. So, the least right she deserves is that you should live with her in good terms and not leave her alone to be afflicted with distress.
We fear that you would be a cause for her to be tempted in her religion and apostate from Islam.
You should know that divorce in normal cases in which there is no risk for the Muslim [i.e. the husabnd] to be spoilt and tempted in his religion, is the most detested permissible action in the Sight of Allaah, and one should not resort to it except in case of necessity or a dire need.
Therefore, we first advise you to fear Allaah and be steadfast on His straight Path, and then keep your wife and have good marital relationship with her, and bring up your children in a way that pleases Allaah.
Then, if you can marry as a second wife your relative whom your family wishes, then all the best, otherwise there is no obedience to a creature in desobeying the Creator; obedience is required only in what is permissible.
Nonetheless, you should be kind and dutiful to your mother with any means. However, it is not permissible for you to obey her in divorcing your pious wife and leaving her at a loss.
The outstanding scholars issued a Fatwa to this effect, among whom were Imaam Ahmad, Shaykh Ibn Taimiyyah and others.
Finally, we ask Allaah to enable us to do good and make us steadfast on His Path until the day we meet him.
Allaah Knows best.