One of the most devastating effects of sin is that it often blinds us to its consequences. Almost every person who gets entangled in a sin like this thinks they will get by with it, and that there won't be any consequences to their actions.
But they are wrong. The Bible's warning couldn't be clearer: "You may be sure that your sin will find you out" (Numbers 32:23). Right now, all your sister can think of is what is happening to her right now and Satan is very good at deceiving her into thinking it will continue forever. The Bible says that there is pleasure in sin for a season (see Hebrews 11:25).
But somewhere down the road it all will come crashing down on her. I don't know when or how but it will happen. The Bible warns, "Do not be deceived: God cannot be mocked. A man reaps what he sows. The one who sows to please his sinful nature, from that nature will reap destruction" (Galatians 6:7-8).
What can you do? First, pray for your sister. Only God can break through the hard shell around her heart, and only God can convict her of her sin and bring her to repentance and faith in Christ. Then ask God to help you warn her of the consequences, lovingly and yet firmly. Above all, point her to Christ, for only He can forgive her and cleanse her, and only He can put her feet on a new path the path that leads to eternal life.