I don't know too much about starting a new one. But my question is, what kind of sorority you have in mind? As others put it well, it will be a tough battle to start one, esp when you're active in two already.
Why not try to make the best out of your own sorority by taking active roles? Also since your friends are in another service org, why not invite them to rush for your sorority?
But of course, nothing can be forced, if you feel that starting a new one would be best for you and your friends, then go ahead. Good luck!