SPOILERS: Everything you wanted to know about this movie explained.
As children, Cindy and Keith have some degree of incestuous relationship, instigated by her natural childish discovery of the pleasure to be found by grinding her naughty bits against his neck in the chicken fight game. It is unclear how far this relationship goes, but seems likely that it went far beyond childish chicken fights, to the extent that Keith may actually be Ryder's father. At the very least, we know that she frequently begged him to play the chicken fight game. At some point their mother discovered, and Keith was blamed.
Cindy moved to California not just to go to college, but to get away, possibly pregnant with Ryder and not wanting him to grow up in that environment. Keith resented that, may very well have wanted to have a family with her or at least be a father to Ryder.
Keith has an incestuous relationship with Molly, who loves the chicken fight game that he taught her. He wonders if Cindy taught Ryder. He has lots of unresolved feels with Cindy, over her leaving, feeling judged, even betrayed by her.
He orchestrates the entire series of events. Molly (happily?) follows all of his instrucions, from pretending she wants only Ryder to tie her shoes to screaming she wants him to take her to the barn, to her first effort at chickenfighting with him, to running out screaming with fake blood on her dress. Keith's vehement refusal to allow anyone to examine Molly is because there is no injury.
Everything that follows is according to his plan. His specific motives may be subject to some speculation, but at the root of it, he wants to get back at Cindy. To do that, he makes Ryder out to be a predator, as he had been viewed as victimizing Cindy back when. A lot of his behavior bears contemplation, such as during the dinner he invites Ryder to. The message he wrote on the car was for both Ryder and Cindy, Cindy was desperate to not let their mother see it...
Keith set up the dinner so that Ryder would most likely show up alone, if at all. What would he have done if Ryder hadn't shown, or if the parents came, or if they'd called? Who knows. But sending Abby out with one extra horse made it easiest and most likely for Ryder to go alone.
He said he wanted Ryder to feel comfortable and know that there were no hard feelings, but that was obviously not true, as he went to great lengths to make him uncomfortable. The mother asked where his parents were, as if they were expected. He put Ryder at the head of the table and Molly next to him. Not sure about the reason for Molly's demeanor when she came out to the table. was she acting as if Ryder had in fact victimized her? The thing about guessing the ages of the girls? Sending Ryder and Molly into her room alone? Coming in just as he approached her to find out what the message on the game thingy was, as if he suspected wrongdoing. Taking him out to talk about the gun.
I think the whole gun thing was to warn Ryder not to mess with him. Not to tell anyone what he either had or was going to figure out about Keith and Molly.
Then he sent him back with Molly. He gave her explicit instructions, she smiled, she liked the game he was telling her to play. Everything she did was what he wanted her to do. Take him to the river, get him swimming, chicken fight to orgasm, then desert him.
When he got home, no mention was made of where he'd been. Nothing was said about Molly abandoning him. Keith dominated the conversation. Made it clear that Molly had chicken fought with ryder, and that he didn't know how to play. Obviously she knew how to play, he had taught her. Acted surprised that Cindy hadn't taught Ryder, after all it was her favorite game.
So basically he was saying, now you know how it feels to be branded a pervert when you did nothing wrong, someone else manipulated you. I am now an incestuous father and it's your fault, you started it all. My incestuous sex slave will do and say anything that I say, and I have a gun, so don't try to mess with me. You're no better than me, you victimized me, I victimized you back.
Mom feels guilty because she feels responsible for what Ryder went through, for what Keith went through, for what Molly is going through. Ryder tells her she did nothing wrong. She was a child innocently experimenting with feelings she didn't understand. Dad doesn't have the first clue what is going on, but you can bet Keith will clarify it for him if anyone tries to mess with him, don't forget the gun, and don't forget that Molly will surely say that Ryder raped her if he tells her to. Everyone saw the blood, everyone thinks he is a molester. So nobody can do anything, everybody feels like *beep* Keith has his revenge and his little sex slave. End of story.