When a family member professes to be a Christian and refuses to repent of known sin, other believing family members must continue to urge them to stop. Our goal in the conflict remains repentance for the sinner, reconciliation with God and man, and restoration to the fellowship of the church and the family. But sometimes those pesky loved ones refuse to cooperate putting us in a very difficult position as family members. We are family, and we are family of God and I believe that family of God has to take preeminence over earthly family. No less than 13 times in the New Testament are we told to love one another as brothers and sisters in Christ, which is our enduring relationship. We must continue to press on out of love for the person and reverence for Christ.
We have been using the case of "Pat" and "Dan" to illustrate our case. Pat a believing woman has hooked up with Dan who is an unbeliever. Despite all the warnings, pleadings, and revelations from Scripture Pat has decided that she doesn't believe the Bible means what it says in this case. She maintains she is a Believer, but thinks you take the Bible too literally. She believes her relationship with Dan is ok because she is happy and he is happy. They are now planning to get married. Pat has moved in with Dan to save money.
In one final attempt, you approach Pat and warn her with what Scripture has to say about living in sexual immorality (1 Cor.6:13, 1Thess 4:3) and you warn her about being yoked with an unbeliever (2 Cor. 6:14). When you are confident there is nothing more you can say and you are having no real impact you can take a further step.
In the name of our Lord Jesus, when you are assembled, and I with you in spirit, with the power of our Lord Jesus, I have decided to deliver such a one to Satan for the destruction of his flesh, so that his spirit may be saved in the day of the Lord Jesus. 1 Corinthians 5:4-5 (NASB)
Especially when it involves some kind of sexual sin. While all sin is serious and an affront to God there appears to be a special emphasis placed on sexual sin because of the intimate nature of marriage and how the Church is the Bride of Christ (Eph. 5:22-33)
At this point, turn her over. Let her go. Take your hands off of her life and allow her to go down the path of destruction she has chosen for herself, eyes wide open and heart laid bare. Not out of anger or revenge but because this is the pattern set our for us in the Scriptures.
What this looks like is breaking off all social connection and keeping each encounter one of a continuing urge to repent. No casual conversations, no lunch dates, and no rescuing. You must allow her to experience the full weight of the sin life she has chosen. All the while praying for her, that God would grant her the gift of repentance and reconciliation. This is a very hard thing to do, and you will soon realize your own faith and trust will grow in the process.
She may or may not repent, and she may indeed marry the unbeliever and reap a harvest of misery as a result. Your ministry of prayer and consistently reminding her of the love of God and His willingness to reconcile her to Himself upon her repentance is the best ministry you can now have in her life.