"Immediate younger sister" simply means your younger sister. The word "immediate" in this context emphasizes the direct relationship of being siblings and highlights that she is younger than you. It doesn't necessarily imply anything about the age difference between you two.
Essentially, it's just another way to say "younger sister," often used for emphasis or clarity in specific situations. It avoids ambiguity if there might be confusion about whether you're referring to a stepsister, half-sister, or sister-in-law.
