Dean and Sam came to a time when their lives were completely different. In the original timeline (which is the only one they remember), they accidentally killed their younger sister when they were young. Their mother had died in their sister's nursery, and the yellow-eyed demon killed their father. In this timeline, their mom, Dad, and sister are alive. They remember the other timeline but not the new one they're in. Their sister, who died when she was seven, is fifteen, living with their mom.