What should we do regarding our father, who persists in his forbidden relationship and refuses to end it, and how should we deal with our paternal uncles who are concealing this matter? And is it obligatory to inform the woman's ex-husband about her forbidden relationship?

If what was mentioned about the father having an illicit relationship is true, then he has committed a grave wrongdoing. Cooperating to guide him is a commendable act and among the greatest forms of righteousness towards him. It is recommended to supplicate for him frequently. You should continue advising him kindly without harshness. If he does not respond, then leave him, while continuing to supplicate for him frequently, as enjoining good upon a parent differs from enjoining it upon others.

If the uncles concealed this with the intention of covering for him and trying to guide him, then they are doing good. However, if they assisted him in sin, then they are partners in the sin, and they should be advised to repent without severing ties with them. Rather, it is permissible to abandon them if there is a prevailing benefit in doing so.

As for the woman, she should be advised and reminded to fear Allah. If she does not desist, she can be threatened with informing her guardian. Her فسق (open sinfulness) abrogates her right to custody according to most scholars, and efforts should be made to revoke her custody if she does not desist.