>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.

Step sister?
Or is that too obvious.