Can a person sell the inherited property in Mohammedan law without...

My late father in law had inherited few acres of land. He passed away in 1999. In 2015 his tenant has filed a case against my mother in law saying my father in law had agreed to sell a portion of his agricultural land for certain consideration. He has produced a piece of paper said to have been signed by my father in law, though it is not true. Now I want to know whether a Muslim can sell the ancestral property to a third person with out the consent and knowledge of his major children? kindly clarify. Is it not void from the beginning (Voidab..) since the property in question is not self acquired property of my father in law.