shocked!!

so i went in my sisters room to try and help find her expensive earphones. i open her draw and find a clear blue pregnancy test in there....😨😨 and it said on the packet that theres 2 in there but when i opened it there was 1....😨😨 shes 17 nearly 18 and had been in a stable relationship for 2 years. now i feel awful about how to approach the subject to her as shes my little sister and i know i shouldnt of been going through her draws but like i said i was trying to help her find her headphones. she started her period a few days ago (i know that cos i witnessed it in the bathroom when she asked me to bring her a pad) and shes on the pill and has always used protection so maybe she just wanted to be on the safe side and just check anyway? i dont know i just feel like an awful sister that she couldnt come to me to talk and i just dont know what to do now cos i know i was in the wrong here but i dont want her to feel like she cant talk to me 😔