The most common and most likely answer to this crossword clue is the 11 letter word STEPBROTHER
I've seen this crossword clue in The Guardian
S
T
E
P
B
R
O
T
H
E
R
Buddy explains!

The straight part
We know that "Relative" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relative = stepbrother.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.
What does STEPBROTHER mean?
STEPBROTHER noun- The son of one's stepparent who is not the son of either of one's biological parents.
- The stepson of one's parent who is not one's half-brother.