Since, as Shelacious said, the NPHC is not a regualtory arm, there is no "NPHC rule" that would be broken per se. HOWEVER, as I stated (I thought) when I replied, it is against the rules of Delta for a woman to be a member of an NPC group AND Delta Sigma Theta Sorority, Inc. IN OTHER WORDS, it would be up to the rules of the individual NPHC orgs whether or not this is permitted.
SoTrue1920, I know u meant individuals.