There are a number of multi-collegiate, multicultural sororities. The "multi-collegiate" part may not mean that many colleges, but there are others out there.
I would suggest a search of the internet, a thorough perusing of this board, and actually talking to members of several different sororities - NPC, NPHC, local and other - to get to the real reasons women join and stay in sororities.
You could start here: http://nationalmgc.org/
This organization doesn't cover all the various such groups, but it's a start.

They have a GAP chapter in NYC. If TNX is an organization that she's interested in, she can visit the organization's website.

Ehh...I wouldn't point someone in that direction. NMGC is a bit sloppy.

The site is still a little sloppy, but the NMGC recently underwent a complete restructuring, so things are drastically better. Theta Nu Xi is now a member organization (as of December).

OK. That's good to hear :)

I know which one you're talking about. It's true that it's mainly Asian dominated, but they are really trying to push for more diversity. If you go to their NYC website and look up some of their past classes, there is definitely non-Asian diversity there. I think if you joined, you could really help them out with creating a truly multicultural experience. Really, it's one of the well-known multicollegiate sororities in the area; you're going to be fighting an uphill battle if you want to establish one of your own that does the exact same thing as they do.

PM me for more details.

EDIT: Just realized that this post was a month old. Wow, I fail.