Summary

Although there's some misdirection during the final act, at the very end its revealed that the brother and sister had incestuous sex many years ago and had a baby. When they visited to get Tommy to sign the papers to sell the farm to the oil company, they tried to convince him that their dad, Josiah (play by Robert Patrick), raped her, but the final scene indicates that the story Josiah had told Tommy was true, that it was in fact consensual sex between the siblings that produced the child. The baby is killed and buried. I think the part they told Tommy about Josiah abusing them was accurate (e.g. him taking an iron to Tommy's ear), probably incited by his wife's suicide. It's also revealed at the end that Josiah is actually a ghost talking to Tommy, and that the three kids killed him 23 years earlier.

Here's some bullet points of the highlights:
-The brother and sister had an incestuous relationship when they were young
-The mother killed herself when the daughter got pregnant
-Josiah became a mean drunk (I think) and started abusing them
-The three of them (maybe with Tommy merely watching) killed the father
-The siblings killed the baby, probably as soon as it was born, and buried it
-Traumatized by it all, the daughter had her tubes tied after the baby was born
-Tommy started seeing Josiah's ghost when he moved back to the farm
-Tommy dug up Josiah's body, along with the baby, which are charred
-Josiah's ghost saw a vision from his wife about how to cleanse everybody of their sins
-With the siblings back at the farm, Tommy murdered them and the tree catches fire

Parts of that have to be assumed because it's not fully fleshed out, but that's the gist of it I think.
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