What’s turning brothers to terrorism?

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BRUSSELS, BELGIUM-- There's an eerie similarity among the world's deadliest terrorist attacks. The common denominator is not just ISIS. It's rooted in family. The recent attacks in Brussels were carried out by brothers Khalid and Brahim El Bakraoui.

Two brothers also took part in the Charlie Hebdo Paris attack. Six of the 19 hijackers on September 11th were brothers. And, who can forget the Tsarnaev brothers in the Boston marathon bombing?

This got us wondering. What turns brothers into terrorists?

Houston psychologist Dr. Michael Winters explains, "If I've always looked up to you and seeing you do this, that may break down some of the resistance (so) that I say, 'Wait a minute. That's not okay.' But if my brother's doing it, it must be the right thing."

The brotherly bond can be strong, with older siblings having great influence over the young, like with the Tsarnaev brothers.

Terrorist groups have admitted they spend nearly as much time looking for spies as they do plotting their attacks, so bringing in two or more family members cuts down on the possibility of infiltration. "The family bond is gonna make even a tighter connection," says Winters, "than maybe two people that have the same belief but don't have the family bond."

And it's not just brothers but sisters, too. 16-year-old twin sisters Salma and Zahra Halane fled Britain last year to marry ISIS fighters in Syria. Then they tried recruiting the rest of their family to join them.

Greek philosopher Antisthenes once said, "When brothers agree, no fortress is so strong." It seems terrorists groups are taking advantage of that bond. And we're all suffering for it.