Adoption son and inheritance property rights

Hello all, My Grand father was adopted son. We don't hold any documented evidence of the same but on various lad and his educational documents he added the adopted parents name. In the adopted parents only widow mother was there when adoption happened and she didn't had any male/female child during that time and there after. My Grand father died in 1999 and when died before that divided the property on my Mom, dad and uncle's name. My father has two sisters and their marriage happened during late 70's. My grand father was owning that adopted mother's land which he later divided to only his male child's. Query:- 1) My Aunt (Father's sister) trying to claim the land which is our custody as inheritance property and claiming for her? Does this allowed by law? 2) My Grand father died during 1999 and my Aunt's got married still does the 2018 Hindu family succession law give then equal rights? 3) Does the adopted son's same as my grand father who has not earned any property but received from adopted mother does such property can be called inheritance property? And successors can claim the rights on it? 4) What are the legal ways by which I can protect the land division to my aunts?