Is the marriage of a man who had sexual intercourse with his fiancée several times during the engagement period, and they have not repented of this sin after twelve years of continuous marriage, valid?

Being lenient in the interactions between fiancés and engaging in khalwa (definition) (private seclusion) is a cause for immorality. Repentance is obligatory and is not through public declaration, but rather by concealing one's sins, provided the conditions for repentance are met. Marrying someone with whom one has committed zina (fornication/adultery) before ensuring the womb is clear (through istibra') is a matter of scholarly disagreement, and it is valid according to some jurists.