Is it permissible to demand the continuation of commission after the death of the client and the cessation of work, then its resumption with a new contract with the heirs?

It appears that you have no right to demand continued commission payments on a contract in which you have no work. The new agreement made by the employers with the client's heirs does not entitle you to claim anything from them, because brokerage is essentially a fee for performing a service, and you did not perform anything in their new agreement.