To become shit siblings, two people must each first take a shit, collect the shit and keep it in typical shit-form and then freeze the shits. After the shits are frozen thoroughly, the two parties will exchange their frozen shits, stuff their prospective shit-siblings' shit into their own asses and proceed to reheat and then re-excrete the the pre-fab log. They are now officially shit-siblings.
Who cares if you're blood brothers? Me and So-And-So are shit-siblings; way cooler, and way shittier. Ain't that some shit?!!!!