next of kin 
noun [treated as singular or plural]
A person’s closest living relative or relatives: the police notified the next of kin...
- The driver's name was being withheld until police could notify next of kin.
- People worried about who would be notified as their next of kin in case of medical emergency are being urged to carry a new card to avoid confusion.
- He said next of kin would be notified before the man's identity was released.
Synonyms
closest relative, closest relation, nearest blood relation/relative, closest family member