Well, this is a multiple part question.
Let's not conflate manners and agreements. There is either a binding agreement or there isn't.
Asking a third-party seller to run an item does not give you dibs on it. By its nature, the sale is public, and everyone has the right to go after it. The analog analogue is when you are going through a 3600-count box at a show: that doesn't give you dibs on the box.
Once the buyer of the lot contacted you to make a deal, it does not sound like you got to the point of an offer and acceptance, so there is no contract. That said, it would be good hobby etiquette to allow you a shot at the item before it goes elsewhere, just as it is good hobby etiquette to allow a person to finish looking at a row of cards before starting at the other end of it. However, family takes precedence over etiquette. If the winning bidder's brother really wanted the item and you don't have a binding agreement to purchase it, peace in the family takes priority. After all, you don't have to sit down at Thanksgiving with him for the next 30 years and hear about how you screwed him out of the item in favor of a stranger
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