As for the question that a man who has sisters married to polytheist men. When he accepted Islam, he called the sisters and their husbands to believe in the Oneness of Allah and his sisters answered him while their husbands refused. Should he separate his sisters from their husbands or what should he do?
If they (the sisters) are Muslims, then their marriage is invalid, and he should separate his sisters from their husbands, as he is obliged to do so. However, if they were in Islamic lands, the ruler should separate them from their disbelieving husbands. Yet, if they were disbelievers, such as Jewish, Christian, or idolatrous women, their marriage is then valid, but if they embrace Islam, they would then be forbidden to remain married to their husbands who persist in disbelief.
For Allah (Glorified and Exalted be He) says, They are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them.
They must be separated from their disbelieving husbands, unless the husbands embrace Islam during the time of `Iddah (woman's prescribed waiting period after divorce or widowhood), since they are legally still wives of those husbands (during the `Iddah). This is so even after the passing of the `Iddah, according to the correct opinion, if the wife does not get married (to another person), the husband has the right to take her back to his authority just as the Prophet's (peace be upon him) daughter, Zaynab (may Allah be pleased with he), came back to her husband Abu Al-`As ibn Al-Rabi` after he embraced Islam, although by then her `Iddah had expired.
May the peace and blessings of Allah be upon our Prophet, Muhammad, his family, and his Companions.