Oh my gosh!

I can see why you are upset, talk to her without nagging to remind her that she isnt doing right and keep on and on praying for her!! Maybe later she will meet someone else and she will get back in church. If she will let you, take her child to church with you but it may cause problems with her boyfriend so they may not let you. Everyone has a right to learn about God. God will handle this in his way ,.just keep praying

Before I rededicated my life to Christ, I was like your sister. I said I was a christian and I defended my faith but...I wasnt following Chrsit I was walking in sin. By the time I got refocused I had 2 kids out of wed lock and their father and I were on again of again relationship. While we lived together for the longest 4 months of my life that was when my bf became a hard core alcoholic. After I told him he had to leave and we broke off the engagement (Iwas pregnant for the 2 time), I started to change my life. If my parents had told me "get rid of that loser" "youneed to straighten out your life" I would have never gone back to church. Instead, they were there when I needed them, they didn't put me down, judge or lecture me. I went back to church on my own. Love her, encourage her in the good she does and pray for her. Don't push her or she will push away. Pray!Pray!Pray!

Be patient with her. After my divorce, I was the same way. And the more people lectured me, the more stupid  stuff I did. Pray a lot.. A LOT. I'm sure that was a lot of the reason that things changed in my life. But at some point, the things that I wanted became more God focused instead of sex-focused. Make sure your life is up to par as well, cause she will look at your life.

Try to her but if she don't take it in then all you can do is pray for her.  Who knows, this might be her testimony when God brings out.