Talking to my younger sis about :/?

okay, lol so my sister is 15 and is going into high school next fall. and my parents hasnt givin her the talk, idk if my mom even talked about puberty with her. so my sis has a bf, and she has been spendin a lot of time with him, and i have walked in on my sis making out with and him, and his hands where all over her , so if my mom isnt gonna give the talk, then i think i should. i think this talk is wwaayyy past due, so how do i bring it up? i share a room with her, but we dont really talk about this type of stuff, but we are close. so what do i say without it being awkward, and so im not lecturing her? shes not the type of girl who is just gonna ask? and we are going on a ride to my older sisters house, who lives like 1 and half away. and because she has soccor, i am taking her and meeting the rest of our family there, so it will just be us in the car for that amount of time. so should i do it than. when she gets in the car should i jsut be like okay were talking about ? or is that gonna scare her away and jsut ignore me? how should i bring it up?? like how do you think the convo will go? im sorta nervous. haha