What is the religious ruling on a widow who commits adultery with her husband's brother, and what must she do to rectify her sin before God?

Whoever falls into error must repent to Allah Almighty by feeling remorse, abandoning the sin, and resolving never to return to it. Allah Almighty has promised to accept the repentance of those who repent. Whoever falls into this error must conceal it and not inform anyone about it, for the Prophet, peace and blessings be upon him, said: "All of my nation are forgiven except those who publicize their sins." If it is possible for the woman to marry this man after her repentance and after her iddah (waiting period) if she committed adultery, that is better to avoid returning to sin.