You could do a pass the gavel. They would have to give one event (social, philanthropic, fundraiser) they would like to see as a sisterhood and one event they really enjoyed from the past. If you had 2 sisters take notes on large poster boards, you would have the ability to build your calendar for the next semester from their suggestions. (and if there are events that aren't even brought up, well, maybe it would be time to table those for another group in the future.) If they choose the events, maybe they'll be more likely to attend and less likely to complain. Yes, it will take time, but if you keep everyone focused, you'll have a really good idea what is important to the sisters.