My younger sister is very disapproving about me being a lesbian, despite the fact that everyone else in my family is okay with it. What she doesn't know however is that her best friend lost her virginity to me, and that we have had sex several times after that.

More than once while I still lived at home they would have sleepovers at our place, and she would walk in to my room in the middle of the night, we would have sex and than she would go back to my sisters room.
And since I moved out she still occasionally comes to me for sex.

Nothing makes pussy taste better than knowing how pissed my sister would be if she found out.