My advice is if you have hooked up with a frat boy, do NOT mention it to any of the sisters. There was one case where a PNM was talking to one of the sisters about a guy from XYZ fraternity that she had hooked up with the night before (mention nothing about hook ups...it makes you look sleazy). When the sister asked the PNM for the name of the guy, well, it coincidentally happened to be the sister's boyfriend.
DeltaBetaBaby is right in saying do not talk about any of the guys in the fraternity. I have had so many girls be like, "Oh! I hung out with XYZ fraternity this summer! Do you John Smith?! Or how about Joe Schmoe?!" It makes you look like all your interested in are the "guy" aspects of the sorority. Talk up your other interests which have nothing to do with boys.