All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
All praise be to Allah, the Exalted, for guiding you to Islam. We ask Him to bless you with steadfastness and guide your brother to the truth. We advise you to strive in reminding him that Islam is the true religion and that Allah does not accept any other religion but Islam, which is the means to attain deliverance on the Day of Judgment and lead a good life in this worldly life. You should also explain the virtue of this religion and its beautiful aspects to him. You can seek the help of the people of knowledge in the Islamic centers or the like in this regard.
If his wife is Christian, then he is entitled to retain his marriage with her with her current attitude. He is not obliged to divorce her; rather, he should strive in inviting her to Islam. If he cannot remain with her, then he is allowed to divorce her and marry another woman. He should not lose his religion for the sake of his wife; verily, on the Day of Judgment, nothing will be of benefit to him except his Tawheed (belief in the Oneness of Allah) and his righteous deeds.
Allah knows best.