Property issue, purchased from my uncle

Hello Sir, This question is regarding property ownership and rights on property. My grandfather had three sons. My elder uncle, My dad and my younger uncle. Now the matter is, My Grandfather had some property on which all three brothers jointly opened a manufacturing unit in 70's and firm was registered on all three brothers name. in early 80's my grandfather transferred this property to my elder uncle in his will before his demise and other 2 brothers had no objection on it. (Signed registered document of property transfer to my elder uncle name is available). In early 90's my elder uncle sale this property to my mother to purchase another property. All documents related this purchase including sale deed is Govt. registered and available with us. That time firm was doing well and my father continues same Manufacturing unit with my uncles. In late 90's due to huge losses and no scope, firm was closed. After my elder uncle's demise his son and my younger uncle are harassing us and claiming on property land and forcefully run there own business. We asked them they had share in firm but not on land because property owner is my mother. Some months ago after my father's demise now they are more aggressive and trying to threatened us. My mother is objecting and ready to take legal action on them. Now the question arises is - 1. If my elder uncle sold his land to my mother. Why his son claiming on it after his demise. 2. This land was never registered on my younger uncle's name. Although his name was in firm and firm is closed now. Why he is claiming on land in associate with my elder uncle's Son? What can be corrective action by my mother. My mother is senior citizen and they are harassing us by not leaving land after my father's demise.