I said this once to a friend and she fell out laughing. I couldn't figure why it was so funny, I mean he doesn't call me by name he has always called me "sister" shortened over time to "sis". How it all began I'm not sure but probably along the lines of "go over by your sister" or "go see your sister" the kid probably just plain thought it was my name.
whatever the reason, happy 39th birthday to my little [but very big] brother.
sister loves you.