"I'm running late and I have to pack my bag for work tonight at the strip club... wait, what are you doing in my room?  I told you to stay out of here!  Um... this bag?   It's nothing, just some lingerie.  Ugh, fine.  I'm a stripper, ok.  Just don't tell anyone, especially our parents, ok?"  If she wants you to keep her naughty little secret, she's going to have to pay up... with a hot ride on your hard cock!  Who knew your sister looked so hot being a stripper?!  She's totally grossed out, but fucks you anyway since you promise not to tell.  You embarrass her to the fullest by making her dance for you, then strip and then ride your cock until you cum a huge load inside her!