why does george lie to the boss about the reason for lennie mental limitation? why does george claim that he and lennie are cousins?

George needs to come up with a reason for Lennie's limited mental capabilities. Mentally challenged people were not understood well in this era. The boss needed a reason he could understand about Lennie's mental shortcomings. George also needed to convince the boss they were related. The boss would not understand two men travelling together and George taking care of Lennie in another context.