When I was sixteen I walked in on my thirteen-year-old sister getting kissed by a girlfriend. Actually, Stephanie had her hand down my sister's panties too. I promised to keep quiet (and I did) if they would both undress for me.
So they did. I got a boner which they asked to see (and I was more than happy to show them). I asked them to spread their pussy lips so I could see what they had. We did this quite a few times. I eventually masturbated for them.