The Lone Star Baby has called her sister "Sissy" for as long as she could talk - "Sissy" was one of her earliest words. I think it may have been awhile before she figured out that her Sissy even had another name and it was years before she started addressing her sister by her name sometimes. For awhile, though, she has been almost as likely to call her sister by her name as by Sissy, with Sissy still the slightly more frequent form of address.
In recent months, though, the Lone Star Baby has started calling her sister "Sis". I'm not sure where this came from - maybe the fact that we have read the part in the Fudge books where Fudge starts calling his brother "Pete", but I'm not sure. She almost always says "Sis" now, though - it's rather adorable.