This one was in a work of fiction I am reading. In this part of the book the therapists of a brain trauma victim are trying to guage how far back his cognitive skills have bounced.

You meet a girl, the puzzle goes. She tells you she has a sister with the same mother and father and the same birthday, including the year. But she says they aren't twins. What's the deal?

If you have heard this one before, please don't blurt it out right away. Control yourselves, folks <g>. For those who haven't heard it I think it's a nice puzzler. I sure didn't get it right away. I thought it was a trick question, which it isn't.