WASHINGTON (AFP) - The fact that Said and Cherif Kouachi, the two men accused of carrying out the deadly attacks at French satirical magazine Charlie Hebdo, were brothers should come as no surprise, experts say.

"The history of terrorism is replete with pairs of brothers and sisters operating together," University of Massachusetts Lowell criminology professor John Horgan told AFP.

"It's actually far more common than people realise." Radical groups may actually aim for recruiting siblings to fight side by side.

There were three pairs of brothers among the 19 hijackers involved in the Sept 11, 2001 attacks. In November 2012, the Qazi brothers were arrested for planning a bomb attack in New York.

After Mohammed Merah killed seven people at a Jewish school in Toulouse in March 2012, his brother was arrested as an accomplice. And the three Duka brothers were jailed for life in the United States for plotting to kill soldiers at New Jersey's Fort Dix.

"We have plenty of examples across the terrorism spectrum, of multiple kinds of terrorism in Indonesia, Northern Ireland, Middle East," said Horgan, who previously directed a centre for terrorism and security studies at Pennsylvania State University.

More recently, the Tsarnaev brothers carried out the deadly bombing at the Boston marathon finish line, with the youngest facing a possible death sentence at his trial now under way.

And in Canada, the Larmond brothers were arrested Friday on terror-related charges.

These same blood ties linked the two men who stormed into the Charlie Hebdo offices in Paris with guns this week.

Said Kouachi, 34, and Cherif, two years younger, gunned down 12 people before being killed by police after a two-day manhunt and a hostage standoff.

"Together, they formed a hermetic bubble, a molecule of extremism," said University of Maryland psychology professor Arie Kruglanski.

Between brothers and sisters, "there's a social connection that reinforces realities." The two brothers in Paris "reinforced the ideology (of jihad), because they're close to each other. They reinforced the goal to terrorism: you're going to be a hero, it would bring you glamor and glory," added Kruglanski, who studies terrorism and violence.

Both brothers were on the US terror watchlist. Said had gone to Yemen, where he attended Al-Qaeda training camps, while Cherif was convicted in 2008 for involvement in a network sending fighters to Iraq.

Usually, the older sibling tends to influence the younger ones, psychologists say.

But for the Kouachi brothers, Cherif appears to have been more radical. The younger brother was arrested when he tried to leave for Iraq and then further indoctrinated by a radical Islamist in prison.

Regardless, siblings undeniably have an impact on each other.

"They'll have had some similar experiences, such as marginalisation, lack of assimilation," explained Jennifer Jenkins, psychology professor at the University of Toronto.

"As they talk about those shared experiences, they will be influencing how the other views such events," she said. "I think they could certainly fuel one another's experience of marginalisation (and) the need for retribution."

As with unhealthy habits like smoking, drinking and other delinquent behaviors, there's a "role model effect" between siblings, said Susan Averett, an economics professor who studies sibling relationships at Lafayette College.

Whether it is because the younger one simply wants to emulate his or her older sibling, or because the older one consciously entices his or her younger sibling to the behavior, there is a clearly observable effect.

To escalate to a massacre like the one seen this week at Charlie Hebdo, the perpetrators likely held a serious grudge over a social failure or humiliation, the experts said.

But the "special bonds between siblings, that you don't see in non-siblings," means they are "less likely to change their minds at the last minute, that commitment, secrecy are maintained," emphasized Horgan.

That's one reason why jihadist groups are often eager to recruit siblings.

"There is no question that using siblings is very effective, very attractive for terrorist organizations," added Horgan.

Another factor in favor of siblings: intelligence agencies often have more trouble finding them, because siblings communicate frequently and naturally.

And the sibling bond is so powerful, "virtually unbreakable," that the risk of betrayal is minimal, Horgan said, meaning the attack is that much more likely to reach its bloody conclusion.