Like someone else said, if the people who are leaving are just dead weight and are making the whole mood of the chapter negative, they really need to go. Don't only talk about the problems but do something about it. Ask people what they expected from the sisterhood and what are they doing to make sure they get what they wanted from it. A lot of people like to complain about what is wrong, but very few people do anything to actually change it. If people actually committed to spending time with each other on a more social level, then some of your problem will be fixed. Try small group interaction, because sometimes when the group is too large, people have a tendency to just talk to the people who they are use to talking to.
I hope your chapter can find the sisterhood. It might take a while but don't give up.
