Are her children legitimate after she married her relative and saw him with another woman before consummation, and will her repentance be accepted if she deviated then repented after seeing her husband's deviation?

The children of a couple are legally attributed to the husband, even if the husband committed adultery, because the dispute regarding the prohibition of the wife due to adultery with her mother or daughter is considered a doubt that averts punishment and establishes lineage. If the wife participated in the sin, she must offer sincere repentance, for Allah forgives all sins, and "the one who repents from sin is like one who has no sin."