Im worried about my sister/ how do i react?

okay, dont shoot me down straight away, but she left her phone on the counter this morning and there was no one in the room so i thought i'd take a cheeky browse. shes 15 and im just a couple years older and i as i read some messages i ended up finding out something i should probably never have found. it turns out shes had sex and is probably pregnant and is asking her friends for advice and booking a clinic appointment. i was completely shocked at this, and i have no idea what to do? i dont really speak so her because she has some stuipd superiority complex so its hard to communicate with her, do i just pretend i havent seen it? shall i tell my parents? what do i do?