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 is His slave and Messenger.
What this man has committed is a major sin. He has committed Zina (adultery), if he had done so deliberately. In fact, he has cheated and exploited a new Muslim woman who does not know much about Islamic rulings. It was supposed that this man helped his sister adhere to the Sharee’ah and guide her to the truth instead of cheating her to trap her and to have illegal sex with her. So, this man should repent to Allah for what he has done. A true repentance consists of deep sorrow for violating the rulings of the Sharee’ah and being determined not to return to such a sin again. Know that the marriage as mentioned in the question is invalid but the woman is entitled to take the full dowry (bridal money). She also should observe a waiting period (three menstrual periods) and then they could remarry if they wish while fulfilling the conditions of a valid marriage. Now, we advise this brother to renew his marriage contract with that woman and continue his life with her to encourage her to follow the Islamic Sharee’ah and to make up for the sin he has committed against her.
Finally, we state that if the sister had approved of the marriage after having the knowledge of the conditions of a valid marriage, then she too has committed a major sin. If both of them were not aware of the conditions of a valid marriage, then they are not considered sinners any longer.
Allah knows best.