There are several synonyms for the term 'kid sister.' Some commonly used replacements for this term include 'younger sister,' 'little sister,' 'baby sister,' or 'sister-in-law' (in cases where the sibling is married). Additionally, the phrase 'baby sibling' can also be used to refer to a younger brother or sister. Although these terms may differ slightly in their connotation, they all refer to a female sibling who is younger than oneself. Choosing the right term to use may depend on the context, such as whether formality is required, or the specific age difference between siblings.