Could I have some examples in the novel from start to finish how George and Lennie's relationship can be compared to one of an older brother and a younger brother?
George is very protective of Lennie and certainly shows signs of being a benevolent older brother in the way he 'looks out' for his simple friend. We see this from the very start of the novel, "Lennie, for God's sakes don't drink so much....You gonna be sick like you was last night" (p.8). Despite the exasperation and trouble that Lennie brings to George the smaller man still has deep attachment to Lennie. It comes both from fondness due to the longevity of their relationship and a sense of duty from the commitment he made to Lennie's Aunt Clara.
George tells the story of how he accepted responsibility for Lennie before Lennie's Aunt Clara died. This happens early in the book. George articulates their relationship in this way, demonstrating his dedication and promise to look after Lennie, despite the obvious challenges this entails (regarding George's patience, his social life, and his independence).
George does act like a protective older brother to Lennie, and Lennie looks up to George. George looks out for Lennie, telling him how to behave and what to do and what not to do. He also encourages him by telling him they will one day have a ranch where they can tend rabbits.
Lennie often asks George if he is mad. He wants to please George, even though he sometimes can't control himself.
"I ain't mad at you. I'm mad at this here Curley bastard. I hoped we was gonna get a little stake together- maybe a hundred dollars." His tone grew decisive. "You keep away from Curley, Lennie." (ch 2)
George is constantly worrying about Lennie. He is afraid that he will do something or say something to get them kicked out of work or run out of town, such as when Lennie wanted to touch a girl’s dress. Of course, George’s fears are well-founded. In the end, he cannot protect Lennie from others and he cannot protect others from Lennie, so he shoots him.