Good call Gene...
How about, "Honey, can I talk to you about something? I am worried about your sister....You see, the other day, in her room, I unintentionally stumbled across a naked picture of her. I hope she isn't in some kind of trouble."
Or, "I should have told you this sooner but I was afraid you would get mad at me, I don't know why...but the other day, I saw your sister naked."
Or you could use it as a way to sneak by a major cigar purchase "Guess what...I saw your sister naked the other day, man is she hot...I mean BOINGGGGGG....oh yeah, I alse bought $1,000 dollars worth of cigars." that way, she'll be so mad about her sister, she won't even think about the cigars.