I agree with you, Tranq. The cost/benefit probably isn't there. And, with the murkiness concerning how the money came to be where it is, I ventured the constructive trust possibility only because the OP has hinted that auntie has acknowledged at some point that the money was intended for the brother and auntie had assumed a "trustee" role.There is no attorney who is going to take this over $5,000. I still don't have a legal argument on why the person deserves the money by legal process at all. Constructive trusts are fancy law and it would be extremely unlikely (we can't even guess because we don't know the state) on these facts. Most cases I've seen in my state are quite clear about what was going on and what happened.
Still, the family relationships seem to be fractured already and a bluff in the form of a letter to auntie is low cost and might work.