1. in a manner involving sexual relations between closely related family members
Usage: clinical/legal context
2. in an excessively close or insular manner, especially within a group or organization
Usage: figurative

Examples

• The small community operated incestuously, with the same families controlling all major businesses.
• The academic department was criticized for hiring incestuously from within their own graduate programs.
• The board members were accused of making decisions incestuously without outside input.
• The publishing industry sometimes operates incestuously, with editors only promoting books by their friends.
• The political party’s leadership structure functioned incestuously, excluding fresh perspectives.
• The art world can be incestuously connected, with the same collectors and dealers dominating the market.