Mom Has Another Daughter

Suddenly, Jon's cell phone rang. It was his mom, so he answered it. "Jon, I just wanted to tell you that the office called me. I have to go in for a few hours. You'll have to get something to eat yourself. And I'll probably be back late, maybe after eleven." "Okay, I'll tell Karyn." "Huh? Why would you tell your friend? She's not staying over, is she? Now, Jon. We talked about this." "Staying over?" Jon asked, confused. "Mom, Karyn's my *sister*. Remember? She lives with us." "What are you talking about?" his mom asked over the phone. "You only have *one* sister: Zoe."