I don't know how much familiarity you have with Burning Wheel, either via reading or play.I'm not sure how any of that makes the introduction of the item not be player fiat. - Actually the last part if true maybe, but it certainly wasn't established in the example (not sure if it was in the actual play). And I think the Beliefs would have to be far more specific about the item in question than just conflict with the brother in general.
Suppose that the player, at the start of the session, wrote a Belief for his PC: "I will recover the Falcon's Claw from my old tower." (And for all that I can recall, maybe that's what happened.)
That will affect what sorts of actions and roleplay earn Artha. It doesn't make any difference to the basic proposition that (i) it is the player who is making discovery of the Falcon's Claw a possible element of play, and (ii) the canonical procedure for resolving that possibility is a test on Scavenging.