This Is Jane, Jim by Kaye Umansky
Jim is not impressed with his new baby sister. She gets all the attention and cuddles and no one seems to have any time to play with Jim. So Jim goes stomping to his bedroom. Crashes in and slams the door. Climbs in bed and hugs his teddy. No one loves him any more. But in the night he hears Baby Jane crying and goes to her room to investigate and in the morning Jim and Jane are asleep together, gently breathing, fast asleep.