Can alumnae attend formals?

[astroAPhi]

Every year we hold our Forget-Me-Not Formal in the spring and I've always been told it's sisters only... no alumnae. I'm just wondering if this is a national rule or a local rule from us (since no one really seems to know or I get conflicting answers)? I'm just curious because I've been to other Greek formals and having the alumni there is a great opportunity to get to know your alumni better as well as a chance to catch up with those that you do know. It's been mentioned that it's a liability issue, so I just wanted to see if it was a national rule, or if it was an old local rule that we must have instilled several years ago.