in ‧ ces ‧ tu ‧ ous /ɪnˈsestʃuəs/ BrE AmE adjective
1 . involving sexual activity between people who are closely related in a family:
an incestuous relationship
2 . involving a small group of people who only spend time with or help each other, not people outside the group – used to show disapproval:
an incestuous political community