An "extended family member" is defined by the law or custom of the Indian child’s tribe, or in the absence of such law or custom, is a person who has reached the age of 18 and who is the Indian child’s grandparent, aunt or uncle, brother or sister, brother-in-law or sister-in-law, niece or nephew, first or second cousin, or stepparent.
25 U.S.C. § 1903(2)