User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-24784846-20150518115847/@comment-9595671-20150518121532

Yeah, his brother died and he wanted to bring him back but never was able to do so. Eventually he wanted to die after he was cursed to be immortal so he made an Etherious demon using Natsu's body hoping his "brother" would be the one to end his suffering.

So basically Natsu's his brother through his body, but his personality is most likely not the same one as his brother's before he died. So he's both his brother and not because as B214 said, Natsu's a demon made by Zeref using his dead brother's body as a vessel.