In Steinbeck's work, George and Lennie are not brothers, but they live as brothers. George tells Lennie they are lucky to have each other. "We got somebody to talk to that gives a damn about us." George protects the mentally impaired Lennie much as a big brother would look after a defenseless younger brother. There is one basic parallel in George's relationship with Lennie and Cain's relationship with Abel. George murders Lennie; Cain murders Abel.
Beyond that, these relationships are more strongly characterized by their differences. Cain slays Abel out of hatred, pride, and jealousy. George kills Lennie out of love, to spare him a horribly painful death at Curly's hands. When confronted by God, Cain does not take responsibility for his actions. When Slim finds George at the river, George does take responsibility. Slim knows what George has done and understands why. He comforts George.
In reference to Abel, Cain asks, "Am I my brother's keeper?" Clearly, he was not. In Of Mice and Men, George is his brother's keeper, until the very end of Lennie's tragic life.