chapter 2 of mice and men
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He says they are cousins and Lennie is slow because he got kicked in the head by a horse.
George explains to the boss that he takes care of Lennie because they're cousins and he promised Lennie's mom he'd take care of him. He explains this because the Boss asks him point blank what their relationship is and why they're together.
I assume this was when George was talking to the ranch boss. The ranch boss thought George was using Lennie and taking his money.