I think the difference between sisters and friends is that sisters have to put up with a lot more. They live with you when the slightest criticism can bring you to tears or you are so touchy you can be tripped as easily as a landmine. They endure your subtle and not-so-subtle insults and pick the barbs out of their skin nonchalantly, as if their skin were actually of a thicker variety than yours.
They look up to you, and then they look down on you from their towering height. They travel far away and settle in other cities, building lives you only have a small window in on. They grow up and suddenly you feel old before your time.
You might bare your soul to a close friend, but no one knows your true heart like your own sister. Because despite your apparent transluncency, you wear a mask at times and a suitable disguise. You shift in and out of character until you are not sure at all about your actual identity. But your sister, she knows the face behind the mask and the wreck behind the false front. She knows, but because she is your sister, she loves you as you are.