Guilty of incest; involving, or pertaining to, incest; as, an incestuous person or connection.  Shak.  [1913 Webster]

 resembling incest; between people or groups so closely related as to make such activity improper or unethical; as, an incestuous coziness between the management of the company and its union; an incestuous relationship between the salesman and the purchasing manager.  [PJC]