By no means am I an expert about this situation (as I am not a member of Tri Sigma OR Sigma Phi Epsilon) -
As others have said, there is no official brother & sister fraternity. The fraternity (or sorority) that you "connect with", for lack of a better term, can change from campus to campus, or even from year to year - the fraternity that you do ALL your socials with as you're in the chapter may become the "chapter no one is seen with" by the time you graduate.
And yes, I've seen this happen.
