If your sister is having sex without a barrier form of birth control like a condom, she is totally at risk for a number of unpleasant things like an unintended pregnancy, many different sexually-transmitted diseases and HIV/AIDS. Using a condom, a spermicide and another form of birth control would greatly reduce the chances of contracting something that is at best, unpleasant, like contracting a curable STD, and at worst, life-altering, like getting pregnant or contracting an incurable STD or HIV.

Of course, you might not know the whole story--maybe she IS using condoms and you don't see them around. Or maybe she didn't use birth control at first and is now using it regularly. But whatever the case, if she is at all sexually active, it's crucial for her to be tested for any STDs, HIV and to have regular gynecological exams.

Try communicating with your sister in any way you can, and make it clear that you don't want to break up her relationship with her boyfriend or jeopardize yours with her, but that what she is doing is very serious. Maybe you can try to approach her when she's not stressed or preoccupied with something else. Consider bringing up the issue in a non-confrontational way. If she won't listen to you, maybe you can write her a letter or an email or give her written material (even this column, if you like) explaining the dangers of unprotected sexual activity.

If your sister still won't listen to you, consider whether you want to tell your parents about it. You might even let your sister know that you're thinking about telling your parents and see how she responds. She might be more likely to hear you out. While it may feel like you're ratting her out and she could be angry at you for doing so, her health is seriously at risk and so you may need a serious solution.