Is a man's marriage to a woman with whom he had an illicit relationship before marriage, and who was previously married to another man (thayyib), Islamically valid, given that the husband is aware of this and did not inform the shaykh of her situation?

If the relationship before the marriage contract did not reach the point of fornication (zina), then it does not affect the validity of the marriage. However, if fornication occurred, and then he married her after her repentance, the marriage is valid, and the omission of mentioning her repentance does not harm it. If he married her before her repentance, there is a difference of opinion regarding the validity of the marriage, but the child is attributed to him. Due to the corruptions that result from annulling a marriage after consummation, there is no harm in acting upon the opinion of those who validate such a marriage. Both spouses must repent and conceal their sin.