This silence on the subject, says Dr. Dennis Kimberly, who counsels incest survivors, is part of the reason sex between siblings remains so disturbingly common.

Speaking at the Guelph sexuality conference earlier this summer, Kimberley, a Memorial University of Newfoundland professor, said that sibling incest is far more common that father-daughter incest.

But often, parents find it hard to draw the line between kids just being "yours looks different than mine" curiosity and more inappropriate sexual behaviour between siblings.

Is a brother and sister taking pictures of each other naked and posting them on the Internet incest? What about a brother who sets his sister up to have sex with a friend? In both cases, no sex occurred between siblings, explained Kimberly, but there was still sexual exploitation.

And while once is more than enough (about one-third of sibling incest cases are one-time occurrences), the real problems develop when the activity continues for months or even years, explained Kimberley.

How does it happen?

In some cases, said Kimberley, you have a highly sexualized child, again, a tough call since most children are sexually curious and expressive.

Though safe to say that while self-stimulation isn’t uncommon for a two-year-old, explained Kimberley, rubbing yourself the point of bleeding as one little girl he encountered did, may be a sign of bigger problems.

SUB: Perilous parents

Children whose parents are unavailable may overly attach themselves to a sibling and the sexual intimacy follows, explained Kimberley. Or if children share a childhood trauma, they may develop a bond that becomes sexual.

Kimberley told of a four-year-old boy who saw his mother raped by his father on the kitchen floor and went on to develop an incestuous relationship with his brother.

Sometimes, like some kind of unfortunate psychological heirloom, the pattern gets passed along generations. One of Kimberley’s patients suffered abuse from his grandparents and cuddled with his sister for comfort. When sis started developing, that comfort included letting him touch her boobs. The guy grew up, married a woman who’d also been abused as a child, and had a daughter they named after his sister. When his wife withdrew sexually she told him to go have sex with the daughter.

It gets worse. Kimberley has seen a number of cases in which parents have taught their kids to have sex with each other for their own gratification. In one case, the parents filmed their kids having sex and invited others to watch. The brother and sister obliged, ironically, as a way to bond through the horror of it.

Not all sibling sex is quite so gut wrenching.

Sometimes, it happens innocently as with one young man who had sex with a schoolmate only to find out the girl was his sister from his father’s secret affair.

And in some cultures, it’s not unusual for brothers and sisters to sleep together and learn about sex from each other, explained Kimberley.

In fact, go online and you’ll find plenty of chat rooms and support groups for consenting sibling sexual relationships. Some even believe it’s an orientation, much like homosexuality, and encourage practitioners to "come out of the closet."

NOT TO BE IGNORED

The problem with this analogy, said Kimberley, is that beyond the risks of inbreeding (supporters claim the risks of genetic defects are exaggerated) and the social taboo, human family bonds (incest happens throughout the animal kingdom) are much different than the bonds we create with non-family members.

As a result, the risks of post-trauma and psychosexual problems from an ongoing sexual relationship with a sibling are much greater.

One of Kimberley’s female clients had a brother who used to signal his readiness for sex by rubbing her arm. As an adult, every time her partner rubs her arm it triggers memories of incest. A male client could only have sex with his wife if she shaved because his sexual relationship with his sister started just as she was developing pubic hair. Some patients suffer a condition in which their partner's face actually morphs in their minds into that of the brother or sister they had sex with.

Sadly, in most cases, parents who discover their children are sexually involved often think, "Hey, it’s no big deal. It’s only their brother or sister, not the guy next door or some stranger jumping out of the bushes. Let’s move on," explains Kimberley. "They figure if they simply stop the behaviour, the problem’s gone. They aren’t prepared to deal with the potential long-term harm."

Or the disruption to the family.

Which is why victim blame is common in sibling incest, explained Kimberley.

"It can be painful, because it destabilizes the family. Some victims have to disconnect from the family in order to heal, which can be upsetting to everyone," he said.

Though not nearly as upsetting as suddenly seeing your sister’s face when you’re having sex with your wife I don’t imagine.

For more information and resources about sibling incest visit Survivors of Sibling Sexual Abuse online at www.sasian.org.