Many parents who are covertly incestuous come from traumatic childhoods themselves where boundaries were non-existent and they were often the victims of the same type of treatment. Often this occurs when there is a divorce, death or mental illness where one spouse is unavailable to the other to fulfill their normal adult needs.
This is where the damage can begin. The parent can begin to rely upon the child to fulfill those needs and the child naturally obliges, in part because instinct tells them if the parent’s needs aren’t met, then the child’s own needs will not be met and partly because the child feels “special” and as though they have been “chosen” by the parent to be their confidante and partner. It’s a confusing and very harmful dyad that is absolutely detrimental to the child and usually results in numerous issues well into adulthood.