https://www.quora.com/Does-I-have-a-baby-sister-mean-I-have-a-youngest-sister-and-my-sister-is-not-a-baby-or-I-have-a-sister-and-my-sister-is-a-baby-when-considering-them-without-a-context
“Baby sister” simply means a younger sister. She can be a baby, a child, or an adult, but she has to be younger than you. (Initially, though, ...