20 C.F.R. § 404.363
If you are the insured's stepchild, as defined in § 404.357, we consider you dependent on him or her if you were receiving at least one-half of your support from him or her at one of these times—
[44 FR 34481, June 15, 1979, as amended at 75 FR 52621, Aug. 27, 2010]