how can someone view my facebook page when i set everything to "friends only"?

[vl_hj_hs]

somehow, my girlfriends sister saw something that she weren't suppose to see but im not even friends with her and somehow she screenshotted that image on her iphone and sent it to my girlfriend so both of us were wondering how the heck did her sister even saw my posts when i'm not even friend with her and my whole page is set to private from outsiders. can anyone give me any details? we spent over and hour trying out different ways to see how she can see my page but everything we've done is the same, PRIVATE. and my girlfriend doesn't use her sisters iphone also so pretty sure that my girlfriend didn't log into her fb on her sister's phone. please help???
no, there were no tags nor she even commented or liked it.
no, there were no tags nor she even commented or liked it.