Adoption valid?

Hi All Experts in Forum,

I have a simple question regarding civil dispute for a property between my grandparents.
There were 3 sons to granfather's father. First son (A) had no issues. Second(B) and my grandfather(C) are sons of second wife. My Grandpa(C) was living in his sister house from childhood.  Second grandfather some how managed to get the Will done in his name from first grandfather(A). A died long back. Then my grandfather asked share in the property after realizing property is given to B completly by A.

Second(B) Grandfather claim they have given my grandfather(C) as adoption to his sister.But there are no evidences to proove that. Only will executed by my grandfather's sister for giving her property to my grandpa.

Is this the sufficient evidence to prove that my grandpa is adopted son ?

This case now reached Karnataka high court after passing through lower courts.

Please suggest.....

Thanks in advance