My advice is to answer their questions about your own house, but do not volunteer any more information than necessary (providing this is going to be a bad situation). Maybe just agree with them when they say their sorority is so great, yadda yadda yadda...hey, everyone thinks their sorority is the best, after all! If things aren't going well, try to steer the conversation in non-sorority direction?
When I deal with people who go off on an anti-Greek rampage, I just sit there, listen to them, then nod and say, "Not all organizations are like that, but I can see where you are coming from." If you can let them feel like their opinion is worth something, instead of counteracting, chances are they will just shut up.
The sad thing is sometimes educating people doesn't work if they refuse to hear the other side of the story.
