Once a principal is reported to have died then the POA executed even jointly by all the principals, stands automatically cancelled especially if the POA deed is not clubbed with interest.
If at all other principals want to continue with this power agent then they may have to execute a fresh deed in favor of the power agent.
If the earlier sale deeds were executed, you may go through the relevant sale deeds that who executed the sale deed, i.e., whether the power agent or by the principals.
Section 17 of the Act describes the document which compulsory to register. Those are as follows: Instrument of Gift of immovable property; Non-testamentary instruments which are purporting to creation, assignment, declaration, extinguishing of any interest in any immovable property Lease of immovable property for a period exceeding one year or rent is paid yearly;Contracts for transfer of immovable property for a consideration as per Section 53A of Transfer of Property Act, 1882 which is executed on or after the commencement of this act.
If you are one of the principals, you may issue a legal notice to the agent stating that since the power deed is revoked upon death of one of the principals, he is no more a power agent and he cannot carry out the tasks assigned to him through the POA deed anymore and he should not indulge in any such activity which are detrimental to the interests of the property owner failing which he can be prosecuted under the provisions of crimninal law for the offences of cheating, land grabbing, and other fraudulent activities.
After that you can proceed with the further actions as per law.