The most common and most likely answer to this crossword clue is the 3 plus 3 letter word OUR KID
I've seen this crossword clue in The Times
O
U
R
K
I
D
More clues leading to the result OUR KID
Buddy explains!

The straight part
We know that "my younger sibling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case my younger sibling = our kid.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.