My brother wants to have a copy of my file since he wants to start the game where I'm at, but I don't know how to copy my save. Is it possible to copy my Assassins Creed 3 save file onto another xbox account/profile or even on the same profile?
You could use something like Horizon. It's basically a modding tool for Save Set users, basically 1000/1000'ing games, I would recommend using that but then the temptation to start illegitimately completing games may become too much.
You would put the save on a USB, open Horizon on the Desktop, open the save, HAVE an Assassin's Creed III save made by your brothers Xbox, do some stuff, like assigning your brothers Profile ID, Save and Rehash, and bob's your uncle.
But... Like I said this is basically discussing Modding and that's not allowed.
Knifegash: If Anya spent just one day with me, you can go ahead and call her Anya Plowed.