Name In 7/12

I have a father's plot, which after his death I added my mother's and my name, but did not add my sister's name as she does not stay in India. This was done in mutual consent from my sister. Now I require to sell the plot and an agent who is aware that I have not added my sister's name is threatening me that he will file a complaint to the registrar about having a sister and name not added. I would like to know that can a third person take such action and stop the sale of the plot.