I am so sorry for your loss.   A listing agreement is a contract between the owners of the real estate and the listing broker.  As any contract, the agreement must be signed by the party against whom the contract is sought to be enforced in order for that party to be bound by its terms.  If a party does not sign it they can not be bound.  So the answer here would be yes, all parties must sign and ultimatey sign the contract for sale and transfer of the deed.  I would like to nkow how the deed is presently held.  Your siblings can rty and legally force the sale (called ana ction for partition).  I would speak with a lawyer yourself as to options.  Good luck.