My Gf's Naughty Sister, Zazie

Waking up horny and alone is no way to live. That's why I decided to pay a visit to my girlfriend who was living with her parents - and I was hoping her folks will not be home! Well, turns out that even my girlfriend was out - but her sister Zazie was very much home! Apparently my girlfriend talked about how big my, well, tool is, and Zazie can't think about anything else ever since! And now she wants to see it! What can I do? I'll take it out and I'll let her play with it, and I'll hope my girlfriend will not be coming home...