Hi Wayne,

Above everything else, my advice in this situation is to be gracious, without condoning what they are doing. Your sister probably knows already what is right and wrong but has rationalised what she is doing. One thing about people is that our hearts can rationalise anything that our heads want to do (Jer 17:9). If this is the case, then she is not going to agree with anything you say to her, especially if you do it in a judgemental way. If she is, as you say, ‘backslidden’ then caring for her (and praying for her) is the best thing you can do.

That being said, what is it about having sex that she is not prepared to wait for? Does she fear loosing the guy if she waits (or stops)? Do they think they love each other so much that they have to express it? I would try to find out the answers to some of these questions and go from there.

The Bible does have quite a lot to say about sex before (and outside of) marriage: especially on the nature and purpose of sex. It is a gift from God to be shared between one man and one woman in a life-long, monogamous relationship (Gen 2:24).

Jesus affirms that this verse is in fact talking about sexual union when he teaches about divorce (Mark 10:6-8) as does Paul when talking about sexual impurity (1 Cor 6:16). Sex is intended as the ‘glue’ that holds a marriage together - it unites you to the person you are having sex with. It is therefore only rightfully expressed within marriage.

It should also be obvious that sex makes babies. In fact, this is part of what it was designed to do (Gen 1:28). Even in the modern world with various means of contraception there are many ‘surprise’ pregnancies for various reasons (I was one). Is your sister ready for that?

Finally, marriage is supposed to be a ‘metaphor’ for Christ and the church (Eph 5:22-33). That is, people are supposed to be able to look at marriages and see in them a reflection of the way God loves his people. Sex outside of marriage clearly damages this picture.

As you pointed out, I think the first thing you can do for your sister is to pray for her - especially her upcoming marriage. Marriage is hard enough without bringing guilt into it over what you did with each other before it even started.