Don't beat yourself up over it, because at your age you are going through your sexual Awakening and your sister is your main standard of feminine beauty. And if the roles were reversed she would have looked at your pictures because you have the same blood in you and that's why your sister had those nudes in her phone in the first place you're both sexually curious in a little adventurous. You had no ill intentions and at least you could rest easy on the fact that you have good taste in women because you felt a little guilty about it which means you respect your sister as a person and now she's on your sexual radar and it's just another chapter that you're opening in your life. Someday you'll tell her about it and she'll probably laugh or feel flattered and she might even be a little disappointed that you didn't do anything to the pictures LOL. This is just part of growing up and remember every young man is told to go out and find a woman just like dear old mom and marry her and your sister probably resembles your old mom as well so it's not your fault that they didn't add into that tradition that at some point you would have to sexualize the woman that you're going to marry as and you're going to have to think about your mother in that role as well and that's a big thing to leave out of that Homespun saying that they also tell girls about find a guy just like dear old dad and it can cause a lot of conflict and inner turmoil. So the fact that you reached out on here shows that you've got some strength of character