Speaking with total honesty here, I wouldn't go there..
You are not responsible for your sister. You aren't her parent, her boyfriend or her legal guardian, so it isn't up to you. Furthermore, people, especially younger people who are still finding their independence, don't always react reasonably to being confronted by their family members about problems. It may be that part of the problem is that your sister has been lectured so many times about unprotected sex that she's just burned out on the fearmongering.
I know it's difficult when you're concerned for someone and you really want them to make the right choices, but at the end of the day you can't control what they do. Everyone knows that unprotected sex is risky in terms of pregnancy, so simply reminding your sister of that doesn't seem likely to have the desired effect.
I mean, if you have a good relationship with your sister and she sees you as someone who she can talk to about stuff, then you might try steering the conversation towards it at some point. Since you haven't met her boyfriend, you could try asking her about her relationship and how things are going and see if she's willing to talk to you about it, because there's going to be some reason. Maybe she wants to get pregnant, maybe she doesn't think having an abortion would be a big deal, maybe she's insecure about her relationship and doesn't feel comfortable asking her boyfriend to wear condoms, maybe she's scared of the side effects of going on the pill. Until you know what the problem actually is it's hard to really know what to do or say to fix it.
