NO NPC SORORITY HAS AN OFFICIAL "BROTHER/SISTER" RELATIONSHIP WITH A FRATERNITY ON AN INTER/NATIONAL LEVEL. Sororities may have relationships on a campus by campus level, but no NPCs have an official relationship with a fraternity. Having one may jeopardize the single-sex status (according to what I've read).

The only official relationship I know of is Zeta Phi Beta Sorority, Inc. and Phi Beta Sigma Fraternity, Inc. Members of the NPHC, they share a constitutional bond.