That's so funny that you're referring to it as sororities....I had this problem when I had dated a frat guy!
Once I dated him I was "off limits" to all his brothers, even the ones who hadn't pledged yet!!!
If you didn't have anything too serious with aX, then go for it. If you did, then DON'T DO IT!!! I had a friend who was in xxx sorority, and she and her boyfriend of 6 years broke up (yep, they went to college together from high school), and like 2 weeks later he was taking another sister of hers to their formal, they were officially an item, etc. My friend didn't even really care (he was bad for her anyways), the rest of her sisters were the ones who made a big stink about it.
I also got reamed by a sister because I talked to (read, conversation with, not talked to as in dating) a guy who she had dated like the year previous and she was still in love with him and I had never even known they dated (they dated before she joined the sorority). So I don't know. It just depends on the situation, I guess.
Hope that helped!!!