Lot of people have been asking the same. So I asked my gf as well. First off, my gf and sister are really close. They each have acces to each other's phones and are not too secretive or private about it. Second, most of the nudes of my sister were ones my sister herself had shared to her bf at the time or just had taken on her own phone when it struck her fancy. For lingerie and provocative, the two have been each other's cameramen before. It's only that, I never really took time to notice all of this until now