My Grandmother and her sister always called one another "Sis." They might have taken up the Sis thing because their names were Doris and Devern. Wouldn't you prefer Sis?
My lasting memory of my Great Aunt Devern is her sitting in an easy chair, rocking, talking, and reaching over to a table for a snack. We all watched her pick up a box of dog treats, pour several into her hand, and slowly munch them one by one. As kids we were mesmerized and after she left we almost died from laughter.
Maybe my Sis needs dog munchies instead of Chocolate of the Month Club treats for her birthday?