ACK! (((((((((((((((Candy))))))))))))))))))

Breathe. There's still time for her to change her mind. Perhaps by greasing the wheel and talking to your sister and other family members to convince her to stay elsewhere. Or make it REALLY clear you're not going to be helping her out. You've got a responsibility to yourself first and foremost.

Did she give a reason for wanting to move there specifically? I don't know... she could be trying to "fix" the problems from the past and she thinks that will be accomplished by living closer?

Or she just really likes the area where you live maybe? I hope she changes her mind. But don't freak out about this now... there's still time. It's not "fall" yet.