Shigaraki wasn't 'brother material'. He didn't fit the category for much of anything with his nihilistic ideals.

You were another one of All For One's strays. Someone he picked up a little after him.

You were always going to be used as a decoy. They were just saving up for it, but as plan after plan went down, AFO began urging Shigaraki to finally put you into motion. Something felt off about this, yet he proceeded, dragging you away from Toga.

"Brat, get over here."