My 18th Birthday Gift, A Cock. What Do I Do With It?

My big sister Vanessa gave me a cupcake for my birthday from my most favorite bakery. I didn't expect a gift from her at all. She also gave me a little vibrator with a remote control. I've never seen one of these before. She tells me to put in down my panties on my clit. I told her it feels funny down there. She keeps asking me questions about my sexual experience. I keep telling her that I haven't had any! I mean, I'm only turning 18 today! She can't believe I don't already play with vibrators and says that it should make me feel the way I feel when I'm making out with a guy. I told her I haven't made out with any guys. I haven't seen or touched a guy's dick. I wouldn't know what to do with it anyway. She immediately remedies this situation by bringing our step-brother in as a cock model for me. I can't believe she wants me to touch my brothers penis! So gross! I certainly don't want Mom and Dad to find out about this. We all agree this is my 18th birthday secret and none of us will tell anyone, not even our friends.