Flashback: 22nd of August, 1986. 2 months after Jake turned six. "You dare fight back?" The Dark Lord asked. It was midnight, and he had all called us to a meeting. "Yes." William said. "Muggleborns and halfbloods are-" "Silence!" Voldemort said. Mother was sobbing. "No, my first son-no...." Father was looking with a face full of anger and unhappiness. "Avada Kedavra!" Voldemort said. I was six at the time. I was puzzled. Why did Will fall to the floor? Why was mother crying? But what I knew was that I hated him. I hated You-Know-Who. He was treating people the wrong way. When I looked more to the left, I saw him. My eyes opened wide in fear. His face was pale, drained and gray. His eyes were brown orbs staring lifelessly into the ceiling, After we had left, I couldn't cry nor speak. I felt ashamed not to cry because my eldest brother had died. I spent wide awake in my room, under the covers. It was a sensation unlike any I had ever known, a constant weight that seemed to press down on my chest. I couldn't sleep, my mind replaying the horrific scene of William's death over and over again.
So, there was actually the eldest son. His name was William. He was just like Jake, but he spoke it out loud and got what he didnt deserve. So to bring William back at some point, Narcissa named their last child Will after William. So that's why Jake is Jake :D