First of all you should ascertain the legal heirs/successors in interest to succeed to the estates left behind by the deceased property owner.
The so called legal heir is a class II legal heir, hence the family tree certificate even if notarised, may not be held as legally valid especially the bank may not entertain an application for loan on the basis of this notarised family tree certificate.
They may have to obtain a legal heirship certificate from the revenue department or a declaration through court of law declaring the said people as the successors in interest to the deceased.
Further, you may have to obtain, the death certificate and the legal heirship certificate of another deceased legal heir i.e., one of the sisters who is reported to have died.
The relinquishment deed should have to be executed by all the legal heirs including the legal heirs of the deceased legal heir and it is to be executed by a registered document.
The release deed executed on an unregistered document, even if it is notarised is not legally valid, this would render defective title to the seller.
If the seller is having a defective title, then it is not advisable to buy the property until he acquires proper and marketable title to the property.