A wild guess. I’m pretty sure the actual answer is a lot simpler. Are you
Yggdrasil, the world-tree from Norse mythology?
Any info I glanced on the topic I have gathered from the net. Some are bound to be quite stupid. At the very best.
My brother is the King
Odin, who rules over Valhalla, has several ties with the Yggdrasil. Yggdrasil means “Ygg’s Horse” and Ygg is one of Odin’s names; Odin hanged himself from the tree to gain understanding of runes.
I spent my youth growing on heaven
It’s kind of a divinity. (some doubts here)
and may travel the world as an adult,
Its root run all over the nine worlds.
even a king doesn't know how.
While Odin certainly traveled between worlds, Yggdrasil is on all of them at any time.
I'm the only one of my kind,
There is of course only one Yggdrasil.
yet I'm not in danger to get extinct
And, thank Odin, it shows no sign of dying.
and yet lie about my origin.
As a tree, he still lies about its origin. I doubt it’ll ever move.
The true children of my family are sometimes poison,
Some plants are poisonous.
whereas I prefer to use brute force,
But Yggdrasil is not; it’s just as large as the world.
killing the innocent who seek my shadow.
Among the worlds Yggdrasil roots go through, there is Svartalfheim, land of the Dark Elves, who are practice the cult of the dead. (Very far fetched.)
OdinThe Pain comics, Science vs Religion http://www.thepaincomics.com/weekly041229a.htm