Sister and I got naked together
When I was 13, my 15 year old sister walked in on me masturbating with a girlie magazine. We were both embarrassed and I covered up and she closed the door. A few minutes later she knocked and came back in. She said she had heard about guys beating off and wanted to see. I said no first, but then she pointed at the magazine and asked if I would like to see her naked. So I agreed and masturbated as she stripped. We did it often and later she would also masturbate at the same time. All we did was touch and masturbate. She told me it was alright as long as we didn't have sex. I didn't think so, but could not turn it down at the time.
Now I am engaged to be married and while she is the only girl I have had sex with (and I have told her so), I have not told her about my sister. Is that a lie? Should I tell her?