PoA to execute sale deed

Hi, I and my wife are joint owners of our flat in Uttar Pradesh, which we are selling and have already signed a sale agreement with buyer. We are residing in Bangalore, and cannot travel to UP at this point to register the sale deed. We want to give PoA to my wife's brother who resides in UP to execute the sale deed on our behalf. The sale agreement is already executed by both of us, and money will also be transferred to our accounts. The PoA is only to execute sale deed, as we cannot be physically present before the Sub-Registrar in UP for the same. We have below queries: 1. Do we make a GPA or a SPA? 2. Can we make a single GPA/SPA where both me and my wife give PoA to my wife's brother, or do we need to create two GPA/SPAs separately. 3. Do we register the PoA in Banagalore or in UP? And for registering the PoA, do my wife's brother need to be physically present before the Sub Registrar? 4. What is the stamp duty we need to pay to register the PoA? 5. If we are executing sale deed through GPA/SPA, will the buyer need to pay extra stamp duty on the property sale? 6. Since my wife's brother is not my blood relative, will it change the stamp duty we need to pay to register PoA or stamp duty the property buyer need to buy for property sale. 7. Who will keep the original PoA , should we keep it, should we send it to my wife's brother and will the buyer keep it after execution of sale deed?