Birth right on grandfather's property after adoption

After adoption a child is completely taken out of natural family and transplaned into the adoptive family. Once you are adopted you cease to be your paternal grandfather's grandson. But was your adoption correct and legally tenable ? the normal rule is a person above 15 can't be adopted (unless there is an established custom in that behalf) if your adoption falls to the ground then you can inherit as per the will, otherwise not. 

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Thanks for your response..  one small question...

My grandfather clearly mentioned male children of his son in plural (and we are only 2 grandsons).... according to his mind set I am also his grandson... otherwise he would have mentioned Grandson and that belongs to my brother....

Also the property is from my grandfather, not from my father... so he can offer me the property also... Please correct, if I am wrong...

Coming to the adoption reason, my maternal grandfather is too old and he wanted me to do the cremations after his death..

Thanks for your response..  one small question...

My grandfather clearly mentioned male children of his son in plural (and we are only 2 grandsons).... according to his mind set I am also his grandson... otherwise he would have mentioned Grandson and that belongs to my brother....

Also the property is from my grandfather, not from my father... so he can offer me the property also... Please correct, if I am wrong...

Coming to the adoption reason, my maternal grandfather is too old and he wanted me to do the cremations after his death..