The real son got in prison as a juvenile. For the sake of the plot, I suppose, he has been in prison for too long for his family to even recognize him. He tells his cell mate about his life, to the degree that his cell mate is even aware of the most intimate of details. After his cell mate gets out of prison, he impersonates him in order to rob his family. These details were reveled at the end of the second act, I suppose.
From what I recall, the real guy’s sister doesn’t entirely buy it. He kills the father with a non-traceable injection of sorts in order to get his one-million-dollar life insurance (I think he was diabetic). I remember a funeral scene was shown.
When he reveals himself, he forces the sister to approve to withdraw the life insurance money, for which the bank replied that such a transaction would take too long.
I recall a car crash and a burnt cabin or a summer house towards the end. I think the sister was tied up in the aforementioned cabin. A genuine reunion of the siblings occurs at the very end.
I am not entirely sure of all the details mentioned because I watched it as a kid.