Where can I find evidence in the text that upholds the ideas of "sacrifice through brotherhood?"
I tend to think that some of the best evidence for sacrifice in light of brotherhood comes from the association that George and Lennie share. For George, the best evidence of this is that he never leaves Lennie. Consider that the opening of the book involves all of the things he could have done without him, and yet, he refuses to pursue any of these because he knows he is bound to take care of Lennie. Similar evidence can be found when George is speaking to Slim about Lennie in chapter 3, where George speaks to the idea that he made a promise to Lennie's Aunt Clara to look after him. Finally, when George goes out with the guys in Chapter 4, Crooks torments Lennie with the exact fear that he would be abandoned by George. The reality is that George does not leave him. He does return for Lennie. These moments speak to how there is brotherhood demonstrated through sacrifice. On Lennie's part, consider that no matter what he does, he always seeks George's affirmation to do it. When Curley is beating him up, Lennie expresses sadness at crippling him because he doesn't want George to be angry with him. Lennie "follows the script" that George indicates for him and when he does do something wrong, the first words he utters are usually something about how George will get angry with him. At his last moments of life, he is concerned with George being angry at him and hoping that he isn't. These instances demonstrate clearly that brotherhood through sacrifice is an embedded part of the relationship between Lennie and George. Just as their familiar refrain about "Guys like us" indicates, both concepts are interlinked to one another in terms of how George and Lennie progress throughout the narrative.