They are in an environment in which two men can only behave kindly to one another if they have a tangible family connection. During the time in which this book was written, people with mental issues were not understood.
Along with that, two men traveling together and one taking care of the other was not widely accepted. If they both had individual families it would be different, but being that it was just them two, questions were raised. The boss needed a reason to understand why they were together and saying they were cousins was an understanding reason.