20 CFR 219.44: Evidence of relationship of a person other than a parent or child.

(a) Claimants other than child or parent. When any person other than a child or parent applies for benefits due because of the employee's death or because of the death of a beneficiary, the Board may ask the claimant for evidence of relationship.

(b) Evidence required. The type of evidence required is dependent upon the amount payable and the claimant's relationship to the deceased employee or beneficiary.

(c) More than one eligible and claimants agree on relationship. If there is more than one person eligible for benefits, and all eligible persons agree on the relationship of each other eligible person, only one of the persons will be asked to furnish proof of relationship. For example, if brothers and sisters of a deceased employee file applications for the residual lump sum or annuity payments due but unpaid at death, only one of them need file proof of relationship if their applications indicate that there is no dispute as to who are the brothers and sisters of the employee.