My mother had many older brothers and one younger sister. When little sister came along that is what they all called her 'Sister'. She was named for an aunt who lived on the property and was in the house all the time so rather than confuse folks they just called her 'Sister' all along.
When Mama and her brothers started having kids we all heard them talking to our aunt 'Sister' so that is what my siblings and cousins and I called her too. Aunt Sister. You can't imagine how many people have laughed at that one.
She preferred it to her name...Winifred. When she was a grownup her friends called her Freddie. Which suited her. Red haired and right at four feet tall!