Little sister having sex

So I'm 18 I have a boyfriend, but we're both very responsible and don't want to risk having a child. My friend got me condoms for my birthday for this one inside joke (to long to explain) anyways so I just put them in with my period supply's in one of my dresser drawers. I never opened them or anything. Yesterday I needed some supplies and came across them so I figured I should just throw them away but then I noticed two were missing, now unless my dad (my mom passed away when I was little) stole two out of the ten condoms in the package it had to be my sister who is 13, or her friend (13 too) that always comes over. Should I talk to her about it, it could mean a lot of things, maybe she gave them to someone else, I also know that there was a 'rumor' at school going around that she and this guy named DJ had sex in the locker rooms, anyways I honestly don't know what to do, should I just mind my own business? Or do I confront her?