Hey! I am the social chairman for my chapter, and planning sisterhoods is part of my job.

First of all, in the social chairman's manual, there are a TON of great sisterhood ideas.... If you don't have a copy of this manual, it is available on the "Members Only" area of the Pi Phi website (an entire site, in fact, which is very helpful).
Everything I've been hearing from chapter members is that they want to do sisterhood events that don't take up their ENTIRE Saturday, or a whole evening. They want short events that don't involve leaving the campus or even the house! This is totally understandable, because of all the things Pi Phis are involved with these days.

If you keep it short but fun, lots of girls will show up. Also, when announcing the event to the chapter at a meeting or in an e-mail, make it sound like a lot of fun. If you are really excited about it, the rest of the chapter will be, also.
Here are some sisterhoods we've done recently:
1. Right before formal recruitment, we had a "relaxation night" in our house. The girls put on soothing spa music and did manicures and pedicures, yoga, and one of the girls did back and hand massages. It was quick, fun, AND relaxing at a very stressful time.
2. Last Winter, we took a day trip to a nearby ski resort. Granted it did take an entire day so there was a low turnout, but the girls who went had a blast.
3. Themed movie nights: chick flick night, comedy night, horror movie night, holiday-themed movie night, etc. This is an awesome event because no one has to leave the house. Also, invite new members over to enjoy the time bonding with actives, OR invite non-members that you are interested in rushing.
4. Last Fall, we all went to a nearby lake and the planned activity was to create boats out of cardboard boxes and race them. It was hilarious! Our social chair gave out all kinds of silly award for the best decorated boat, etc.
5. We had family ethnic dinner night - every "family" within the house picked a different sort of food (Mexican, Italian, etc.) and made an appetizer, main course, or dessert, and the whole chapter pigged out together!
6. Next month, we are going to a local YMCA camp to do teambuilding activites on the high and low ropes course: ground-bound activities in the morning, and high ropes course in the afternoon for those with height issues.

I'll have to let everyone know how it turns out....
Anyway, those are just a few ideas, but like I said, there are many more in the social chairman's manual and on the website. Hope this helped!