Unregistered plot

my father bought a piece of land .........it is an unregistered POA type of plot bought in 1984 in delhi......now the one of thegrandsons of previous owner ,who himself died about 13 years ago, is claiming right to this place has sent me a notice under section 106 of transfer of property act........the owner had sold a part of his huge holding to my father ......all the other present owners of other adjacent plots also have the same chain of documents as we do.... .but he seems to b targeting me only as i feel i am a woman and stay at noida ....and might appear weaker..... how can i face this once and for all and quickly so that no dispute aises and i can smoothly carry on my plans of developing this place for future use......please help..... the documents are power of attorney affidavit supporting POA will of the owner in favour of my father sales agreement reciept signed by owner this plot is in a colony which was unauthorised before now has been regularised but registery has still not been opened by the delhi govt..... my question is how can the grandson sent me such a notice after almost 30 since we have had this in our possession ......his notice also laments that this place was given to my father for safekeeping......rridiculous claims can i sue him for damages in court and harrasment