Aunt Sis is a term used to refer to an aunt who is also a sister. The term can be broken down into three parts: SIB (3 letters), RELS (4 letters), and REL (3 letters). SIB stands for sibling, RELS stands for relatives, and REL stands for relation. BRO (3 letters) is short for brother, which is another term for a male sibling. Therefore, Aunt Sis is an aunt who is also a sister, making her a female sibling of the same family.