Case Law on Consent Witness.

hopefully see this site may be helpful

supremecourtcaselaw.com

Cheers

Your  sister is the legal heir to the property , if you want to sell the property consent from your sister is also required as a party to the agreement and not as a witness . if you have made an agreement that you will give her the share of the transaction that is mutual understanding which is left to you people

Role of a witness is totally different that what you have presumed (remember he/she is not party to the contract )

Purchaser's concern is valid

please refer   supremecourtcaselaw.com  for any case laws asis a database

Cheers

Dear Mr. Raghavendra Rao:

Thank you for your reply.  The property got sold recently, but not to the builder referred above.  Sister signed a release deed for the above mentioned property.

Best Regards,

Kartheek D. N.