Im facing an issue regarding fraudulent activity done by my maternal family to Us.

let me explain this in detail,

My father was married to his cousin. My father worked in another country.

on basis of trust, my father gave all of his savings to my maternal Grandfather, who is also his uncle, to buy property. 

My Grandfather bought the property and after 2-3 years, without letting my Father know, He sold that property and money was taken by his two sons.

Same year my grandfather passed away and his sons are refusing to pay the money back. 

what could be the solution in the light of Islamic laws. 

Are they obligated to pay back the money which their father had fradulently taken from us?

Well, all would depend on the intentions and verbal or written agreement between your father and his uncle. If he was giving the money to his uncle to support him and his family, he would not be able to claim it back from a religious perspective.

If he was giving the money for investment on his behalf and it was verbally or in writing agreed, than that is his money. His uncle was only his care taker. He is moral rights to take it back. His legal rights would be valid only if he could prove that that money given to his uncle was for the purpose of investment on his behalf.