What is the expiation for a mistake I have made, what should I do while I am in my iddah (waiting period), and how do I repent?

One of the greatest afflictions is leniency regarding the mixing of men and women, especially relatives. You must be wary of them, for the Prophet, peace and blessings be upon him, said: "Beware of entering upon women." A man asked: "What about the hamw (male in-law)?" He replied: "The hamw is death." You were lenient with this man by enabling him to be in seclusion with you, and the sin is graver because you are married or in your iddah (waiting period) after a death. What is incumbent upon you is to immediately hasten to sincere repentance and sever all ties with this man. Repentance is an expiation for these sins.