1. Nothing wrong legally
2. A poa can be given by both owners to their relative for complying with registration formalities on their behalf before the registration department
However do remember the sale deed must be signed by the co owners before the Indian Embassy or consulate in the country where they presently reside
The poa must not contain a power to sell, because then it would be required to be stamped and registered compulsorily. As per stamp law the sister in law of the wife (her husband's sister) is not a specified relative. But she is a specified relative of husband being her sister. So in case power is given to a non specified relative, there is compulsory stamping and registration needed. That is why if wife also gives power to her sister in law, the poa wud have to stamped and registered for the wife's share in the property. To avoid all this, its better if the sale deed is signed by the owners themselves and only registration formalities are done by the poa holder on their behalf
3. As per instructions of the vendors duly recorded in sale deed
The cheque can be handed over to the poa holder (so include a power to accept cheque on behalf of owners) who can then deposit it in the NRO account of the owners maintained with an Indian bank or you can directly do a net bank transfer to the specified account
4. If the owners have provided you their PAN cards ie of both husband and wife, then you can deduct TDS for both. Ideally that should be done.