The most common and most likely answer to this crossword clue is the 11 letter word STEP-PARENT
I've seen this crossword clue in The Guardian
S
T
E
P
-
P
A
R
E
N
T
Buddy explains!

The straight part
We know that "Family member/Family member" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case family member/family member = step-parent.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.
Crossword Answer definitions
STEP-PARENT noun- One's parent's spouse who is not one's biological parent