This code system http://hl7.org/fhir/policyholder-relationship defines the following codes:

CodeDisplayDefinition
childChildThe Beneficiary is a child of the Policyholder
parentParentThe Beneficiary is a parent of the Policyholder
spouseSpouseThe Beneficiary is a spouse or equivalent of the Policyholder
commonCommon Law SpouseThe Beneficiary is a common law spouse or equivalent of the Policyholder
otherOtherThe Beneficiary has some other relationship the Policyholder
selfSelfThe Beneficiary is the Policyholder