Whatever acts of our relatives are, this does not affect their rights. They still owe us sincere advice and the helping hands that take them out of their sorrowful state. When Allah likes to guide a person, He facilitates for them a person to show them the way of guidance and inform them about Allah and His religion. May we be that person who can lend them that hand. Another major element is to supplicate Allah to guide them and forgive them and help them and to help them get out of this bad situation.

Answering this question, Sheikh Ahmad Kutty, A Senior Lecturer and an Islamic Scholar at the Islamic Institute of Toronto, Ontario, Canada , states,

Once we are certain that our brother is guilty of such sins, it is our duty to advise him and make him aware of the consequences of his grave sins.
Once we have done this, then we must leave the rest to Allah. Though if we know that he is going to harm others through such practices, we also need to warn others against the harm he may possibly cause. We can never side with evil. So we had got to shun evil and do not condone evil.
Once this part is done, we leave the rest to Allah.