The most common and most likely answer to this crossword clue is the 14 letter word SISTERS-IN-LAW
I've seen this crossword clue in The Times
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More clues leading to the result SISTERS-IN-LAW
Buddy explains!

The straight part
We know that "Relatives" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relatives = sisters-in-law.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 SISTERS-IN-LAW mean?
SISTERS-IN-LAW noun- A female relative of one's generation, separated by one degree of marriage:
- Co-sister-in-law: The wife of one's sibling-in-law.