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.

Exhibit A:

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.