He stared at her incestously.

The concept of spiritual incest is often used to describe a close, intimate relationship between two individuals within a religious or spiritual context.

She is known for her anti-incest views.

Their incestuous relationship was kept a secret from the rest of the family.

They were living together incestuously.

The family's incestuousness was a closely guarded secret.

Their relationship was deemed incestual by society.

He was accused of being an incestual.

Their relationship was incestlike, causing concern among their friends and family.

She has a strong incestophobic reaction whenever the topic is brought up.