The Collaborative International Dictionary of English v.0.48:

Incesttuous \In*cest"tu*ous\ (?; 135), a. [L. incestuosus: cf.
   F. incestueux.]
   1. Guilty of incest; involving, or pertaining to, incest; as,
      an incestuous person or connection. --Shak.
      [1913 Webster]

            Ere you reach to this incestuous love,
            You must divine and human rights remove. --Dryden.

   2. 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] -- In*cest"tu*ous*ly, adv. --
      In*cest"tu*ous*ness, n.
      [1913 Webster]