User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-24784846-20150518115847/@comment-9595671-20150518131744

OK, let's say it like this.

A person can be defined in two ways, their body and their spirit. When you die your spirit leaves you and is often considered the more vital part of your body.

When Zeref's brother died his spirit left his body and he wanted to pull his spirit back into it, but he never could. When Zeref finally just wanted to die from his suffering of immortality he took his brother's spiritless body and rather than giving it back its original spirit, he created a new one.

So the body of Natsu is Zeref's brother, but the spirit inside of it is not his brother's, its the spirit of someone else that he created.

So in a sense, Zeref is both Natsu's brother and creator. Its the current personality controlling the body that isn't his brother.