Distribution of property after death of grandfather to adopted child

HI,

I will be discussing here some dispute about my property and hope for some valuable suggestions. Problem is like that- My grand father owe around 20 acres of property as per subjected records. Recently, he passed away. It broke up my heart but not much of their own sons.

My grandfather was a very gentleman soft nature and i love him a lot. He has 4 sons mainly A,B,C,D. My Grandfather died without making any WILL which will ensure who will be the owner of his property after him. Talking about his four children A,B,C,D all of them have children's except D. D died after the marriage and D's wife lived single and she adopted a boy of 15 years who is her brothers grand son, she also died by making the will that all her property goes to that boy after her death. Now A,B,C decided to share the property equally among themselves, is it necessary to give property to that adopted child.? A,B,C- alive. adopted boy have own property from their own parents. adoptation also done secretly with out intimating to A,C.

Really confused didn't know what to do and how to step forward to resolve this dispute.