next of kin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person's closest living relative, often entitled to inherit the personal property, and in some cases the real property, of the deceased, if that person dies intestate.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Closest blood relative, heir to inheritance.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the person who is (or persons who are) most closely related to a given person