An incestuous relationship is one between close family members, often romantic or sexual.
When we talk about something being "incestuous", we're usually talking about situations that are unhealthy or unacceptable because of their closeness. For example, imagine a high school teacher who starts dating one of their students - it's not just the power imbalance that makes it wrong, but also how close they are in age and authority. When someone says "this plotline is too incestuous", they mean it's uncomfortable or creepy due to its focus on relationships between characters who are too closely related.
In addition to describing a romantic or sexual relationship, the word "incestuous" can be used to describe situations that feel similarly close or suffocating, but not necessarily involving family members.
Pertaining to or engaging in incest.
Example: If you engaged in an incestuous relationship with a family member, that would make you an inbreeder.
