Property dispute.

If one of the daughters sold her share to another, it is valid.  If she sold total property then consult local advocate having knowledge in property matters for further guidance.

A co-sharer is just a trustee of the undivided property, and unless there is partition in metes and bounds, allocating each co-sharer a definite share, such sale is not legal.  A co-sharer is not entitled to sell all the property.   Registration is different and authorities are not expected to go into intricate legality in such sale, they are concerned with their fees only.  If there is any illegal alienation, before the limitation period, one should file a suit for partition and declaration suit and make the purchaser also as a party.  Contact a local advocate and get a legal notice issued immediately and obtain an injunction before purchaser resorts to some other development of the property.  The sale cannot bind other co-sharers.

The sister concerned is entitled to dispose of her undivided share in the property. The matter could be sorted out with the buyer. Or else, any one of the jointees, the buyer included, can file a suit for partition.