How we cite our quotes: All quotations are from Rain Man.
Quote #4
SUSANNA: Charlie, you're his brother! His brother! They tell you today for the first time that you have a brother, and I don't see in your face one little reaction. I'm not saying joy. I'm saying something.
CHARLIE: Just take it easy—you don't know what I'm going through here.
SUSANNA: What are you going through? What are you going through? Because I don't know. Because you don't tell me anything! You just give me lies! Lies! Lies!
CHARLIE: Lies? What lies? What lies?
SUSANNA: This thing that Dr. Bruner asked you to bring him here, this is bulls***. I know it's not true. So why don't you tell me why, why is he here?
CHARLIE: Because I'm pissed at him.
SUSANNA: At who?
CHARLIE: At my father.
SUSANNA: You're pissed at your father, and you bring Raymond here. Why?
CHARLIE: I don't know why. 'Cause I got him and they want him!
SUSANNA: This makes no sense!
CHARLIE: Raymond was left all the money and I got nothing.
SUSANNA: How much?
CHARLIE: $3 million, the inheritance. Every penny of it.
SUSANNA: So?
CHARLIE: So, I'm gonna keep him 'til I get my half! I deserve that!
Here, Susanna is going after Charlie for not really ever expressing his feelings—and for lying when he does communicate. She's realized that Charlie kidnapped Ray, contrary to what he initially said went down at Wallbrook… so, she's not too happy with his "communication" skills just now.
Quote #5
CHARLIE: Ray, Ray, Ray, Ray, you are never gonna solve it. You know why you're never gonna solve it? 'Cause it's not a riddle, man. And if you understood that—if you understood that it's funny—you know, you might get better.
Charlie and Ray end up bonding eventually, and Charlie finally gets how to relate to/communicate with Ray and address his needs (in terms of communication and everything else). Here, he's trying to get Ray to understand Abbott and Costello's "Who's on first" as a joke rather than a riddle/honest question—and if Ray just "got" stuff like that, Charlie suggests, he could "get better."
His thoughts don't really indicate a deep understanding of autism or how it works (and some might argue the whole movie has that issue), but the point is that he's trying to help Ray get a deeper understanding of language and its nuances/double-meanings, which would in turn help him out socially.
Quote #6
CHARLIE: I like having you for my big brother.
RAYMOND: Yeah. C-H-A-R-L-l-E. C-H-A-R-L-l-E. Main man.
Ray had previously liked to spell out the name of a guy at Wallbrook named Vern, whom he called his "main man." But at the end of the movie, he's started doing that same ritual with his brother's name. He uses the spelling to show a connection to/affection for a person, and it's about as close to saying "I love you" as Ray seems to get.