Registration of documents

Form 32A is about photographs and finger prints of executant,claimant and the witness , maybe your brother had nominated mother as her local representative to take care of his property and also if the money for such transactions is paid by the brother he will always have a legally enforceable stake over such property

At one place you saying that both brother and mom sign the doc and in the last you saying mother signing on behalf of brother , these are 2 different legal configuration

Your mother signed on Form 32-A, but your mother did not sign the document, and hence the document is valid.

I verified further actually the sign of seller as a executant of the sale deed has more weightage and since you have given your acceptance by signing on form 32A therefore the sale itself is complete , sign of buyer as such not required because it's seller who is transferring the ownership in your name and your presence recorded in form 32A so nothing to worry.

Form 32A is a document required under Section 32A of the Registration Act, 1908, to be affixed to a sale deed for official registration. It includes passport-sized photographs, left thumb fingerprints, and personal details (name and permanent address) of the buyer, seller, and any authorized representative. This form helps to verify the identity of the parties involved in a property transaction. 

Your brother, as the principal, must authorize a single representative via a duly registered Special Power of Attorney (SPA) to act on his behalf for both signing and presenting the sale deed for registration. This representative would then complete Form 32A and all other registration formalities.

Your brother may prefer to execute Special Power of Attorney, duly attested.

The conveyance deed is executed by the seller. The buyer has only a limited role.