The card signed by all the sisters sounds like a great idea - and make sure you mention that you would like to reschedule before end of semester (if that's possible). I'm sure they will be thankful you let them know in advance before they got things set up.
One of the sororities (when I was at school) was notorious for cancelling mixers with the less-popular fraternities at the last minute if they got a "better" offer.
