(a) The term parent means a natural mother or father (including the mother of an illegitimate child or the father of an illegitimate child if the usual family relationship existed), mother or father through adoption, or a person who for a period of not less than 1 year stood in the relationship of a parent to a veteran at any time before his or her entry into active service.

(b) Foster relationship must have begun prior to the veteran's 21st birthday. Not more than one father and one mother, as defined, will be recognized in any case. If two persons stood in the relationship of father or mother for 1 year or more, the person who last stood in such relationship before the veteran's last entry into active service will be recognized as the parent.