When a woman commits fornication and weds her lover, does the sin of fornication remain or is it removed simply because they marry?

Fornication is one of the enormities the sin of which is removed by repentance. It is not a condition for repentance that the fornicator marries his lover. Rather, repentance is by desisting from such an act, regretting committing it, and resolving never to return to it again. Allah forgives whoever repents whether or not he marries his lover; repentance is not bound to marriage even if decency demands protecting the woman with whom he has erred. If both of them repent and are suitable for each other, then it is preferable that they marry each other.

Allah the Almighty knows best.