Hi there. I am a green Card holder.

My dad's ancestral home and the land was initially divided to him and his siblings by his parents.
My dad bought his sibling's shares of the property long time back...somewhere in 1970s.

He gifted that to my sister few years back and in 2017, my sister gifted that to me.
From what I understand, when I sell the property, I have to pay tax for the profit from the original 'purchase'.
But in my case, part of the property was inherited by dad and remaining was bought (or received as gift) from his siblings with not enough records around.

How do I calculate the profit to do tax payment in US and India when I sell it?

Thank you!