The only reason I can think of to take naked pictures of yourself and videos is if you are sending them to someone, i.e., sexting or even puting them on snapchat. Did you happen to look at her texts as well? Here are some worries that I would have as a parent. One, she has met someone online somewhere and is sending pictures of herself. Could be a child predator. Two, sexting, if she is doing this, is a sex offense and a person who shows those pictures to others can be labeled as a sex offender and have to register as such. How awful would that be.
Your sister probably does not understand the ramifications of these pictures which obviously anybody can see if they pick up her phone. I would talk to her. You are her older brother and while I'm sure you never wanted to see your sister in this state, you have. Try to find out why they are on there in the first place. Educate her about the dangers of having those kinds of pictures. In this day and age of social media, nothing is safe. Anybody she sent those pictures to could post them on whe web for the world to see.
Kind of look at it this way....Sometimes it is a blessing that someone found out. It is a learning experience before anything bad happens.