Writing will on self earned property

Hello team, I have scenario. My mother in law has a piece of agricultural land in her name. This was purchased from my father in law and at the time of registration he has registered under my mother in law name. They have two children(Elder son and younger daughter). Both are married. I am the daughter in law who is asking this question My father in law is mentally unstable. My husband has the original land papers with him. Recently we got to know without our consent one part of the land is alreday been registered under daughter name with out possessing original papers at the time of registration.(Online RTC displays daughter's name). Daughter has a greed to acquire the next piece of land.I guess Son's Consent is not required since it comes under self acquired property. My question now is, Can the property owner write a will for remaining piece of land under daughter name? How will we(Husband and myself) get to know if the remaining piece of land is been written by a will? Is there a way that we get to know if the property is already written by a will?