Despite the 13 year age gap and, don't get me started on size difference, your sister will still raid your closet.
When they say ketchup goes with everything to small children, they mean everything.
They are indestructible, unless it's your fault. In which case, you would swear it was life threatening based upon the screams and tears.
Cats are either very complacent- or the distinct opposite, completely opposed to the new addition.
Despite all the awesome ideas you have when you get them, about how they will be so awesome, and sweet, and love you, you'll still want to strangle them from time to time.
That for those first few months, you'll do darn near anything to keep them from crying because it's so heart-wrenching...then you get over it and realize she will be happy again, despite how "end of the world" those tears may seem.
They can go from those "end of the world" tears to the happiest child in the world in 3.5 seconds.
Update 04/25: Pull-Ups hold an amazing amount of liquid...but that amazing amount is no where near enough to keep the bed from being wet from time to time.
Kids can in fact gain weight on chocolate milk and fruit snacks. ;)