Two decades ago Maryln Schwartz published A Southern Belle Primer: Or Why Princess Margaret Will Never Be a Kappa Kappa Gamma. Like The Official Preppy Handbook, though not quite as informative, the Primer was a decent piece of pop sociology.
Princess Margaret is five years dead, and I imagine most Southern belles have never heard of her. So a slightly updated version of the original has been released: A Southern Belle Primer: Or Why Paris Hilton Will Never Be a Kappa Kappa Gamma.
The reason Princess Margaret could not have been a KKG in the early 1960s was that she could be seen (often) walking and smoking at the same time. Belles of that era did smoke, but not while moving. That particular rule has been relaxed quite a bit, and belles are much less likely to smoke now in any case.
Of all of Paris Hilton's many transgressions against bellehood I am not sure which one was thought to keep her out of Kappa Kappa Gamma. The fact that she never finished high school, much less attended college, would surely have made it difficult.