While it’s difficult to believe that two brothers would choose to blow themselves up together, as Khalid and Ibrahim El Bakraoui did in last week’s attack in Brussels, security experts say there are lots of reasons why terror groups favour members who are blood-related.

The El Bakraoui brothers were among at least four men involved in the Brussels attacks, according to authorities in Belgium. Khalid blew himself up on a train, while Ibrahim and an accomplice died in explosions at the airport. At least one other attacker is still at large, along with an unknown number of accomplices

The El Bakraoui brothers are just one of many sibling pairs involved in terror attacks in recent years. Salah Abdeslam and his brother Brahim were both involved in November’s Paris attacks; Brahim died at the scene while Salah was finally arrest last week.

Saïd and Cherif Kouachi were brothers who killed 12 people in the Charlie Hebdo offices in Paris last year. And two brothers -- Dzokhar and Tamerlan Tsarnaev – were behind the 2013 Boston Marathon bombings.

So, why are there so many family members who become terrorists together?

The answer may lie in the fact that terrorism, by its nature, is forged on tight community, with members discussing and sharing the same ideas and frustrations while isolating themselves from the wider world.

Terrorism expert Alan Bell, the president of Globe Risk International, says another reason may be that, for terrorist organizations, siblings are the ideal recruits. Family members tend to share the same values, feel an intense sense of loyalty to one another, and can often easily influence one another, he told CTV’s Canada AM on Monday.

“Having future suicide bombers as members in a family unit is better because you know they are there for the right reasons,” Bell said.

In many cases, it’s an older brother who radicalizes his younger brother, Bell says. That’s what occurred with the Tsarnaev brothers. At the same time, it’s also often the older brother who dies first, since it is he who is more willing to make the first move and set the example.

The other reason that terror groups prefer siblings is that, when one member recruits a family member into the group, these new recruits are much less likely to betray the others. A lone wolf entering the fray, on the other hand, could be a police infiltrator, or a spy, or someone ready to turn on members if suddenly arrested.

“When you bring different people together and they’re not family, they’re not related, there’s a chance someone could infiltrate the group and report on them and the attack will fail,” he said.

But all the reasons that siblings make ideal terror group members is also what makes them so hard for police to investigate, says Bell.

“It is easier to track lone-wolf terrorists than it is to track family units because when they’re all together, it’s very difficult for (police) to get in,” he said.

Police now know if they identify a new terror suspect, they should next look at that suspect’s family members.

But Bell says going the next step is often difficult because police find it much harder to get information from family members than it is to get info from a suspect’s co-workers, for example.