From Longman Dictionary of Contemporary Englishkid sister/brotherkid sister/brotherespecially American English informal your kid sister or brother is younger than you are SYN little sister/brother British English → kid
Examples from the Corpus
kid sister/brother• You may have heard of his kid brother.• And Terry was her kid brother.• During their youth Jane was more likely to put her weight and invective behind brother Charles than her kid sister.• Most of them were older than I was, and I was like a kid brother, always hanging around.• I met his kid brother Ed, the flight nurse, back in Beckley, West Virginia, their hometown.• Rule two, stop pretending you got engaged to my kid brother for anything but his money.• He has a kid sister in the fourth grade at school.• He told me he asked his 4-year-old kid sister the same questions.• Still the kid sister to him, she thought dismally.• I suddenly realized Bobby was more than just an annoying kid brother who always wanted to use my stuff. 