I think January 2014 is lots of notice! If you can take the lost income for few months, I'd suggest terminating the problem siblings and seeing how that goes for a month or so. Then, depending on how you feel it's going, you can either find a couple more clients, or give your other siblings notice. Just to help you stick it out until September.

Even if you don't think it'll be any better without these siblings, it might still be worth a shot, especially if it's going to be tough finding work after you close. It's funny how a little change can make a huge difference.