The POA is to be executed by the person/s in whose name the property stands. If the property is in joint name, they need to give joint POA or if it is individual names, they need to give POAs in their individual names. In any case, the POA should contain specific details of the property to be sold and should also contain the express authority to you to sell and execute the relevant documents. Such POA should be a Registered Power of Attorney and should be registered at the place of execution and when it is received at your end it should be stamped as per the local law. Better you contact a local Lawyer to draft the POA and to handle the stamp duty and registration of the POA.
Once the property is sold based on the properly executed and registered POA in your favour, the sale proceeds need to be credited to the NRO account of the Property owners ie. your Brother and Sister in Law. Such money credited into NRO account of the property owners can be transferred to them (your brother and Sister in Law) duly submitting form 15CA and 15CB and other documents as called for by the Bank where it is deposited and/or from where it is being remitted.
The second para relates only after the sale proceeds are received.