This fight is not, actually, something Uchiha Tobirama wants to be in. Not really. She'd rather both her older siblings crawled in a hole and never came out, and instead they're trying to kill each other, and she -
She's getting in the way. It's what she does best.
She's much better at it now than she was at barely thirteen.
As Jakkou twists away from Jakuan's blade (formed from Amaterasu's black flames), opening tiny there-and-gone rifts around herself to tear at her twin brother, Tobi flashes forward, faster than either combatant can fully react to, her sword diving for Jakuan's very briefly exposed side.