How does My Side of the Mountain end?
The book ends with Sam's entire family coming up to Gribley Farm to build a house and live with Sam.
When Sam's mother reads the newspaper articles about Sam, which suggest that he is a boy either without parents or a boy with terrible parents, she decides that she and the family should move up to the mountain.
Though Sam is somewhat resistant, he is also happy to have his family with him. He tries to talk his father out of building a house on the mountain, saying that they could all live in trees and hammocks. Sam's mother tells Sam that until he is eighteen, she will take care of him as she feels she should, in a house, and when he comes of age he will be able to choose for himself.
As his parents, they need to give him a home and take care of him—at least until he turns eighteen. When that day comes, Sam can go off and live however he wants.