talking to my younger sis about "stuff"

okay first off i know this is weird, but my mom isnt gonna tell her, and i want her to be safe. second, i know she is taught this stuff in school but still, i want her to be safe! third, if your just gonna leave some weird of rude comment, dint!
okay so my sister is 15, and is going to high school volgende fall, and she is Pretty developed for a 15 jaar old. she also has a bf, and they have been going out for awhile, and the other day, i came home pagina from my B's house, and i dint think she knew i was home pagina but, when i came in she was sitting on the pino with bf, and they were making, and his hands were all over her boobs, and her hands were all over him. so she is clearly getting serious, lol. so i thought i should talk to about sex, and all that. i know i cant make the decision for her to have sex, but if she decides to i want to make sure shes safe. IKD how to ask her, of bring it up. should i be Straight up with, of should i try to get to the subject? we are going a uur and a half car ride, just her and me Toma, be case she has soc cor, so im just talking her to our sisters house and than meetting the family there, so it will just be us, should i talk to her than? im a little nevous, hhaha i dont want to chicken out.