What is the main conflict in What Jamie Saw by Carolyn Coman?

One night, a little boy named Jamie awakens to see the man who lives with them, Van, take his baby sister, Nin, and throw her across the room. Fortunately, his mother steps up in time to catch the little baby.

That's when Jamie's life begins to change drastically. While he is still numb from the shock, his mother packs a few things and they leave the house, never to return. Mom has a good friend, Earl, and he takes them in without questions, in the middle of the night.