Also, what are you doing for "sisterhoods"? Is it a lot of pressure, like a planned event where people are actually doing something? Or is it something like "go to Sarah's house and watch [insert cool tv show here] every week"?
If you're doing more planned, structured events (usually these require acquiring materials/leaving campus), try doing some deconstructed events like hanging out and watching movies in someone's living room. Also, don't make attendance mandatory. Just make it on a day when there's not a lot going on. ETA: I should note that by not making attendance mandatory you're going to have low attendance for at least the first few weeks, but if a small number of sisters get together and are having fun, word will spread. Just make sure everyone in the chapter knows they're welcome and even wanted there. If you're having trouble getting anyone to show for too long, try a different tv show, or a different hangout deal. It could be anything from going to the campus gym together to cooking a big spaghetti dinner once a week to playing video games, as long as it doesn't require too much effort and is fun for most everybody.
ETA2: If sisterhood is pretty strong in a chapter, these things kindof start to happen naturally, because they're things you just do on a random weeknight with your friends. Once it starts feeling natural and stops feeling forced you'll be in pretty good shape.