kristi--
I know what you mean about being in a rural area, but there are some ideas out there to help rural campuses. If your campus is truly interested in helping with these policies, band together your sororities and try to get a local business to be somewhere that you can have mixers. I know that one campus banded together and found a cheap bus company, and as long as all greeks only use their bus company, they get this ridiculously discounted rate. They found local businesses who were willing to allow them to use their facility earlier in the evening, then opening up the facility to the general public around 11 pm. They also decided that having less mixers, but more elaborate mixers would alleviate some of the cost. So they do a lot of four packs and six pack mixers, so that with more organizations, they can pay less for the mixer.
As for the "three sisters makes a co-sponsored function", look at it this way. If it looks like a duck and quacks like a duck, it's going to be thought to be a duck. When someone sees a fraternity having a party and 2/3s of XYZ sororoity are there, people assume that the sorority is co-sponsoring, even if they aren't. By having these "numbers" in place, some groups feel they will be protecting themselves from liability.
I hope things get better for you
