'He's a Cheat'

Warmtrooper, I'm going to throw a wrench in your theory here because I didn't get that the director was trying to imply that George and Ashley had a sordid affair in any point in time. If George was indeed the father of Ashley's baby how exactly was he able to do that if he hadn't been home in three years as Peg said? Did he drive/fly home nine months earlier without the knowledge of any of his family members/friends with the sole intention of knocking Ashley up and then leaving? I think you're wanting this movie to be more of a soap than it actually is. What we are certain of is that the brothers are in disaccord. I don't believe it has anything to do with a Jerry-Springer-Brother-I-knocked-your-wife-up scenario. It was obvious that Johnny felt anger to his brother and inferiority because 1) George had managed to leave, 2) his wife was smart and a high-class piece of ass, 3) and most importantly because he said so himself. Where you ask? Before and after he got shot down by Madeline, he said they they thought they were better/smarter than him and the rest of their family. And not only that but also it explains why he seems to be in perpetually angry with Ashley, granted he does also show he cares for her but I think he expressed his feelings of entrapment when he states something along the lines of if it hadn't been for her, he would have gone to DC in the 12th grade. What he feels for her isn't anger or resentment because she slept with his brother but resentment because he couldn't leave and make something of himself when he had the chance.

Interesting interpretation, but I don't see it myself.

a)Johnny's hateful attitude towards his brother, including throwing a wrench in his face!
and
b)George, not Johnny, staying with Ashley in the hospital after the baby died.

George is the golden boy of the family and both know it. Some of Johnny's issues stem from growing up in George's shadow.

He threw the wrench because he hated that George did what he wanted to, but felt too insecure and inferior to do so. Such as staying with Ashley at the hospital. Johnny has a serious communication issue (he just can't express himself well).

He felt he wouldn't be of any help to Ashley, such as know what to say that would make her feel better, and that frustrated him. George could do it and would do it well, and this upset Johnny. The guy hadn't been home for three years and there he was, comforting his wife and probably saying the right things and all. It was too much for Johnny.

But throwing the wrench was a turning point for both. Johnny got it off his chest and by not reacting, George acknowledged he was a cause of Johnny's resentment. With this out in the open, they now could move on.

c)George's mom saying "there ain't a thing wrong with you" and George laughing in her face (because she's clueless)

I felt his laugh was due to this: "That is the cause of a conflict between me and Johnny." Her unwavering belief that he was "perfect" drove a valley between two brothers.

d) George's mom saying "he cheats" which intimates he may also cheat with women, but not necessarily with his mom's knowledge.

I felt it was her way of telling Madeline she was part of the family now. Up to then, she kept a wall up between herself and Madeline. Admitting George's "flaw" is her way of showing she trusts Madeline enough to share.

was Ashley so clueless, and professing she hopes the child looks like Johnny, completely ignoring the fact it could be George's? Was she that stupid?

Ashley wasn't clueless at all. It's not even possible for George to be the father as he hadn't seen the family for three years.

Ashley simply loved Johnny. Uncharacteristically for an optimist like Ashley, she admitted she preferred the high school Johnny (probably when he wasn't so weighed down by resentment and insecurity).

did Johnny call Ashley the morning when everyone left, and say "we could try again?" If Johnny knew it wasn't his, then why was he trying to perpetrate this
lie? For Ashley's benefit?

Up to when she lost the baby, he never shared how he felt about her being pregnant and ultimately, her.

Remember, she said he had been like this for "two years". She obviously thought her pregnancy would bring them together closer, but it pushed them further apart. She didn't realise (and probably never will) that Johnny felt even more trapped.

By asking if they could try again, he was saying he finally let go of the resentment (which he took out on his brother by throwing the wrench) and that he wanted to make their marriage work this time.

And why did she yell at him right after he said that?

She was just excited and her excited shouting was in accordance with her personality.

Was it his fault that she slept with his brother?

I don't feel she'd be the type to be deceitful. Her world revolved around Johnny, which was - for a long time - Johnny's burden.