How to procced further..?

Children Numbers:- 1)my mother is elder - Lakshmi 2)My mother's younger sister - Hymavathi (3)My mother's younger brother - Thammayya Father 4)Narayana I'm Madhav and my sister Madhu we are 2 children for my mother "Lakshmi" My mother's father No.4 "Narayana" has 3 children, child.No.(1), child.No(2) and child.No(3) and had "1 Acre land near Guntur as property and a house". Out of these child.No(2) i.e.., "Hymavathi" is unmarried and taken care by child.No.(3) "Thammayya" in the house left by No.4 "Narayana". No.4 "Narayana", died without sharing property and without any 'legal will'. So, children.No.3 "Thammayya" gave that property of 1 Acre land to "Development GPA" by taking "Power of Attorney" from children.No.2 "Hymavathi" and they don't even contact my mother child.No.1 "Lakshmi" as we are living in Visakhapatnam after marriage. This happened in 2014 and construction completed by 2017 and "10 flats" got registered on the name of child.No.3 "Thammayya" and his children. Now my mother child.No.1 "Lakshmi" and child.No.2 "Hymavathi" died. Now we came to know about the fraud and got the "Nakal Documents" of that "1 Acre land". According to the documents child.No.3 "Thammayya" didn't mention about my mother child.No.1 "Lakshmi" as a child of No.4 "Narayana" not even name or any particulars about my mother child.No.1. Now, 1 -> Is there any right for me and my sister to file a case on Child.No.3 and on that property..? 2 -> How and whom we have to send Notice regarding this fraud (No.3 "Thammayya" or "Owner of Development GPA"..? 3 -> How to proceed further..?