Where do you live? Are there any interesting places around you, like theme parks, lakeside cabins, etc? Perhaps you could look into taking a cruise, especially if you are within 8-10 hours from a starting port (driving distance). Most cruise lines give huge discounts to group bookings of cabins, and I can recommend Cruise Depot out of Florida (407-831-1577) and Cruises411.com as places that have done well for group bookings in my past. Do any of your sisters have a cabin on a lake or a beach house that you could use? I mentioned theme parks before...I believe you live in Canada, though I'm not sure where. I know that Canada has Paramount's Wonderland, Marineland, La Ronde, Playland, Galaxyland, etc.
Some simpler ideas are baking things, having a dinner party, afternoon tea, mock horse races, movie night to the stars (dress up and use a room with a projector for a big screen), play Lazer Quest (lazer tag) or paintball, craft day, learn to crochet/quilt/knit, go to a school or local theatre production, have a big picnic with classic games, ice skating, a BBQ, pool party (many local hotels will rent you their indoor pool), wine tasting lessons, dessert fondue party, etc.
These are just some ideas. Not sure what will fit for you, but good luck and have fun planning!