I'm a married woman and I have a sister who according to my father is a prostitute. My sister was married to her cousin but then she ran away because she said didn't want him. She said she wants her own life and moved in with some friends that lead her astray. My father disowned her and his got bad photos of the doing Haraam things. She said that was the past now she cleans. Should I talk to her?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no doubt that your sister had committed a great sin by committing adultery.  Adultery (and/or fornication) is one of the greatest sins.  Allaah warned us about it in His Book as He says (interpretation of meaning): {And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way.}[17:32].  

Another sin that your sister committed is running away from her husband and befriending bad company.

However, if she sincerely repented to Allaah and became righteous, then Allaah forgives whoever repents.  Allaah says (interpretation of meaning): {But indeed, I am the Perpetual Forgiver of whoever repents and believes and does righteousness and then continues in guidance.}[20:82].

So she has to perform acts of good deeds in the future.  Sincere repentance requires that she end her relations and friendship with bad people and keep company with pious believing women.  In addition, she has to return to her husband if the marital relationship is still valid.  But if the marriage became invalid, for instance he divorced her, then she has to return to her father's house and stop going out unnecessarily.  You have to help her to do good, by establishing good relations with her, and her father should forgive her and conceal her sin.  He has to destroy the pictures of her so that Satan would not induce her to return to her sin.

It is confirmed that the Prophet said: "Do not help Satan in leading your brother astray." [Ahmad]

Nevertheless, their marriage is still valid if no divorce took place.  If this is the case, we advise the husband to keep her in kindness, after making sure she is not pregnant from adultery; according to the correct opinion of the people of knowledge.  He has to forgive the past as a way of being kind to her and as a sign of keeping relations with kinship.  He has to teach her matters of her religion and help her in acts of obedience like wearing Hijab and not going out except for a need or necessity, seeking the permission of her husband in doing so.

If divorce had taken place, we advice the husband to take her back if there are no obstacles.

However, if this is not possible, then her guardian has to protect her and prevent her from having bad company, and oblige her to wear Hijab.  He has also to find her a pious husband, as Allaah says (interpretation of meaning): {And marry the unmarried among you and the righteous among your male slaves and female slaves….}[24:32].   

Allaah knows best.