Proof for agricultural income

My wife's grandfather had few acres of agricultural land. After he passed away, my wife's father
continued with the cultivation. By way of inheritance, my wife too has a share in that agricultural land.
My father-in-law is planning to add her as a nominee in the Co-operative society (where the agricultural products
are sold) and ensure that some money is transferred directly to her account from the Co-operative society 
when the agricultural products are sold.
This, she plans to show as agricultural income. Does she need any other proof ?