What is the ruling of the Sharia regarding conjugal relations with one's wife and her subsequent pregnancy after she was declared unlawful (haram) – by saying "She is haram for me, like my mother and sister" – for riding alone in a car with a man, while she was forgetful? And what is the expiation for that? And is the pregnancy resulting from this intercourse unlawful?

Prohibiting one's wife is considered zhihar if she is likened to one's mother or sister. If the wife, out of forgetfulness, does what the zhihar was conditioned upon, then according to the Shafi'i school – which is the predominant view – you are not required to pay the zhihar expiation if your wife was heedful of your condition. However, if she was not heedful, then you are required to pay the zhihar expiation before engaging in intercourse. If you engage in intercourse intentionally without expiation, you are sinful. But if it was out of ignorance, there is no sin. The pregnancy resulting from this intercourse is not considered zina (fornication) and the child is attributed to you. You must promptly pay the expiation.