Coramoor, don't take this wrong but I keep forgetting you are a guy LOL.
Anyway, a local sorority can be in a school's chapter of Panhel if the other sororities vote them in so that part is BS. They can be a full member or an associate member depending on how they do rushing/pledging.
As far as the philanthropy - that is so petty I can't even start. Because of this sorority they don't like they are going to not participate in something that HELPS OTHERS??! What's next, I'm not going to participate in philanthropy because one of the guys in the fraternity never called me back.
As far as why the sororities don't like the local, well some people are just stuck up snobs where locals are concerned, or there might be a numbers problem and they feel this group is adding to it. Or it might be jealousy that the local doesn't have to follow all the restrictive rules that they have. Do you know if the local has any plans to seek national affiliation?
I agree with Russell though, all the fraternities should band together and support the local because if only one or two groups do they will get the cold shoulder.