How do I get past this feeling of intense disapointment?

My youngest sister just turned 18, I'm 28. Ever since she was born we've been inseparable and I've always been very open and candid when the topic of sex came around. My best friend got pregnant after being misinformed about birth control methods and I've been on a mission ever since to be clear with my sister. She's always seemed very mature and always answers questions the way... well, the way she thinks I want her to answer them I guess. I say this because a few weeks ago she snuck out on my watch with a boy, who was on drugs and later claimed she let him "touch" her to his friends. When asked why she snuck out she said she "just wanted to make mistakes because I'm a senior in High School now." That is no joke the answer after she tried and failed to give a fake excuse and got caught again. Last night, I discovered that she had started to send racy photos to her boyfriend and talking about letting him "impregnate" her. I just don't know what to do or think at this point. She's still in H.S. They both are. She's not on birth control but even if she was the way they talk about her getting pregnant like it'd be a grand time just has me beyond disappointed. My cousin got pregnant very young and struggled and my sister knows this. She has volunteered at a preschool since her Freshman year so she knows how kids can be. I just don't know how to deal with this. I don't know how to feel or even how to be around her or talk to her. Any advice would be greatly appreciated.