29 C.F.R. § 780.308

The Act does not define the scope of “immediate family.” Whether an individual other than a parent, spouse or child will be considered as a member of the employer's immediate family, for purposes of sections 3(e)(1) and 13(a)(6)(b), does not depend on the fact that he is related by blood or marriage. Other than a parent, spouse or child, only the following persons will be considered to qualify as part of the employer's immediate family: Step-children, foster children, step-parents and foster parents. Other relatives, even when living permanently in the same household as the employer, will not be considered to be part of the “immediate family.”