Is the marriage of the questioner to her husband—who was a Christian and had another wife whom he did not divorce, and then both of them embraced Islam—considered a marriage of fornication? And what is the ruling on his marriage to the other woman?

The marriage of a couple who embrace Islam together is affirmed, and they are not required to renew it unless there is a legitimate religious impediment, such as the marriage being forbidden in Islam, like marrying a close relative. In such a case, they would be separated. However, if the woman is someone whose marriage is initially permissible, the marriage is affirmed. This is supported by the Sunnah (definition) of the Prophet, peace be upon him, and the consensus of scholars. Therefore, your relationship with your husband is legitimate, and his first Christian wife remains under his marital bond and has spousal rights unless a divorce occurs. It is recommended for your husband to invite her to Islam, and you should assist him in that.