Hello,
1). The GPA expired with the death of your father. Besides you need a registered POA to make any transfers.
2) as you have paid a consideration to your father in an agreement to sell you can send a legal notice to the buyer, your brother and his wife and demand the money to be paid. Do not give up possession.
3) You don't need to make any documents or try to make your brother a tenant as your GPA has no validity and besides your moves will be considered fraudulent.
4) Once you have issued a legal notice, file another complaint to the police along with a copy of the legal Notice.