What is the way to repent and return to Allah and break free from a forbidden relationship, given that the woman is married to a man who has neither religion nor morals, and she is unable to separate from him or from her lover, her sister's husband, with children present and no possibility of returning to her family?

Repent to Allah with sincere repentance from adultery, for it is one of the major sins that incur Allah's wrath. Allah Almighty says: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way." (Quran 17:32). Let your repentance include immediate cessation of the immorality, profound regret for what you have done, and a sincere resolve never to return to this sin. Increase in righteous deeds, remembrance of Allah (dhikr), and supplication (du'a), and try to reform your husband, or separate from him if he does not repent.