Partition Suit

Dear Lawyer/Consultants, My grandmother had a property for which she made a will and it is divided according to the will between my father and his younger brother. They also got the partition deed done on stamp paper of 1% of esimated cost of the property at that time. The Partition deed was registered in Land and Development office, Vikas Sadan, New Delhi. The property was divided as front and rear portion bearing same address. Now, my uncle has sold one part of his share to a 3rd person, about which we had no clue and neither have signed any documents. This 3rd person is now saying that he wants to get into a partition suit and want us to make a part of it and want to treat the complete property as one entity, as per him the division is not as per MCD rules and is not applicable,. My questions: 1. If it is a joint property as per the 3rd party, then can he purchase the property without the sign of both the owners? Doesn't this sale stand null and void as neither we were aware about the sale nor we signed anything. 2. If it is not one entity anymore, can he include us in the partition suit and claim his share fromour share? Please help