The note can be found at the front porch of a house in Monongah. The house is on the southern end of the town and has a For Rent sign in front of it.
I've decided to take you up on your generous offer to move in with you and Louis. As much as I hate to be a burden on you and your family, I need to get Timmy away from our dying town. Thanks to Hornwright and his lies, there's nothing left for us here. We'll pack up the truck and head out there after Timmy's last halloween in Monongah Township.
Love you sis, and see you soon.