The transfer of their share by the co-owners in favor of other owner can be done by a registered gift deed or sale deed, the stamp duty involved for both types of transfer is almost the same.
The execution of the registered deed shall be made in the location where the property situated, i.e., before the concerned SRO only.
If some parties are not able to travel to the place of the property location, then they can execute a registered POA deed in favor of their close relative who will execute the said deed on their behalf.
The proof of their ownership what you have mentioned is not sufficient,, they should possess the legal heirship certificate or any other proofs that they are the descendants and successors in interest to succeed this property.
First of all ascertain their entitlement in the property by valid documentary evidences, if they fail to produce the valid documentary evidence then they may not be able to transfer their share in the property to the prospective transferee