HBD Chick has been pointing out that the Bomb Brothers' parents are a mixed marriage: Dad is a Chechen born in internal exile in Central Asia (due to Stalin expelling the Chechens for fighting for Hitler), while Mom is an Avar from Chechnya's neighbor Dagestan. They are all North Caucasus Muslims, but this is a Modern Family, at least by the standards of the North Caucasus.

Similarly, Tamerlan's widow, the mother of his child, is an American girl, daughter of a doctor, who converted to Islam.

To answer the late Rodney King's question: See, we can all get along.

So, I went to Wikipedia to find out who Mom's Avar people are. Wikipedia offered a wealth of information, such as:

During the Khazar wars against the Caliphate in the 7th century, the Avars sided with Khazaria. Surakat is mentioned as their Khagan around 729-30 AD, followed by Andunik-Nutsal at the time of Abu Muslima, then Dugry-Nutsal. Sarir suffered a partial eclipse after the Arabs gained the upper hand, but managed to reassert its influence in the region in the 9th century. It confronted the weakened Khazars and conducted a friendly policy towards the neighbouring Christian states of Georgia and Alania. 

In the early 12th century, Sarir disintegrated, to be succeeded by the Avar Khanate, a predominantly Muslim polity. The only extant monument of Sarir architecture is a 10th-century church at the village of Datuna. 

Of the fourteen names which figured in the geographical part, we only recognized three - Khorasan, Armenia, Erzerum - interpolated in the text in an ambiguous way. Of the historical names, only one: the impostor magician Smerdis, invoked more as a metaphor. The note seemed to fix the boundaries of Uqbar, but its nebulous reference points were rivers and craters and mountain ranges of that same region. We read, for example, that the lowlands of Tsai Khaldun and the Axa Delta marked the southern frontier and that on the islands of the delta wild horses procreate. ... as a result of the religious persecutions of the thirteenth century, the orthodox believers sought refuge on these islands, where to this day their obelisks remain and where it is not uncommon to unearth their stone mirrors. 

Oh, wait, sorry, that third paragraph is from Jorge Luis Borges's short story "Tlön, Uqbar, Orbis Tertius" about a hoax article inserted into an encyclopedia about a nonexistent nation. Sorry. My mistake. Won't happen again.

Fortunately, hbd chick's summary is more comprehensible:

there are only ca. three million people in dagestan and yet there are several dozen ethnic groups there, one of which is the avars. and then the avars, in turn, are further subdivded in 15+ sub-ethnic groups (who knows which one mrs. tsarnaev comes from), which are further subdivided into tribes (tukkhums), clans (teips), extended families and so on. THIS is a clannish society. 

from The Ghost of Freedom: A History of the Caucasus (2008) 

“The Chechens are now considered exemplary of the mountaineers’ historic resistance to Russian rule, but that reputation is only partly deserved. ... The real engine of the highlander uprisings of the nineteenth century lay farther to the east, in Dagestan. The very name of the region — literally ‘the mountainous land’ — is evidence of its central geographical feature: mountains and plateaus cut by fast-flowing rivers. A congeries of distinct languages and customs has long been characteristic of the area, with social ties formed along lines of clans, extended families, and village groupings. The major ethnic groups — the Avars, Dargins, Kumyks, and Lezgins, among others, with none accounting for more than 30 percent of the population — today represent the dominant factions in Dagestan’s precarious balance of regional, ethnic, and clan interests.”

_____

we hear a similar message about the dagestanis (and also learn some more about the chechens) in The Insurgency in Chechnya and the North Caucasus: From Gazavat to Jihad (2010): 

“Research on the *wirds* and *teips* (clans and families) of the Chechens is difficult to collect and the findings generally frustrate the Western desire for order and clarity. But what this research has demonstrated by its inability to draw straight lines is that *there aren’t straight lines* and the Chechen and Dagestani (or Ingush, Kabard, and Balkarian among others) cultures are not vertical structures, or ‘power verticals’ to use the current Russian vernacular. The Muslim faith is a relatively flat hierarchy to begin with, but by looking at the mountaineer culture and its imposition of yet another layer of clan hierarchy on top of the religious one, it is easy to understand why the Caucasians have so much success at insurgent warfare. North Caucasus social structures are perfect for conducting guerilla and terrorist activity because their societies are already a culture of ‘cells,’ and as we’ve seen, cellular organizations with a high degree of loyalty are paramount to insurgencies. Because familial loyalty sometimes trumps religious authority, and because those same clan are often competing among themselves for status and hegemony, those societal ‘fractures’ were — and still are — exploited by the Russians…. 

“This particular characteristic of Caucasus culture is what gives it strength as an insurgency and yet ultimately keeps it weak when it comes time to make the final move toward independence.[24]“ 

“[24] Although Chechnya has been the primary focus of this book thus far, the Dagestanis have been as much a part of this conflict as anyone else. As of this writing, there are more attacks taking place in Dagestan than in Chechnya. The internal dynamics of Dagestan are even more fractured than Chechnya. Aside from the religious and family aspects, Dagestan is made up of more than 13 different ethnic groups — of which the Avars, Dargins, and Lezgins still comprise less than 60 percent of the population. In addition, there are Laks, Tabasarans, Rutuls, Aguls, Tsakhurs, Kumyks, Nogais, Azeris, Chechens, and Russians, and another 40 or so tiny groups numbering only about 200 total — and they all speak their own language — making Chechnya and its Vainakh cousin Ingushetia look downright homogenous.“ 

clannishness is a strength. and at the same time, clannishness is a weakness.

Here's my guess on the internal family dynamics that, over the last half dozen years led Mom and Tamerlan, followed to a lesser extent by Django, into fervent piety (leaving the secular Chechen Dad behind), and in turn leading Tamerlan and Djohar into pan-Islamist terrorism.

Tamerlan was a high testosterone guy, a boxer, maybe a wise guy. (Similarly, younger brother Djoker was captain of the high school wrestling team.) Like most young men, Tamerlan was looking for a team to lead in the fight against other teams. (A continuing theme here at iSteve is that much of what we think of as politics and war is just male team-oriented aggression, which some groups bleed off more into sports while others go more for good old-fashioned bloodshed.)

As a half-Chechen, Tamerlan inherited a world-famous tradition of national rebellion. His younger brother bore the name of the first President of the breakaway Chechen republic in the early 1990s, a secularized Muslim who had risen up to be a general in the Soviet air force. 

However, a general pattern in Chechen history going back to the 19th Century is that when rebellions get serious, Chechens tend to turn to Islam to patch over tribal divisions and reach out for help from other Muslims beyond their own small population. In the 1990s, for example, rich Saudi Wahhabi fanatics flooded Chechnya with money, helping convert what had been more or less a nationalist rebellion against Russia into an Islamist one.

Likewise, Islamism is also a logical endpoint for diverse mixed-marriage families like the Tsarnaevs. Dad is a Chechen but Mom is an Avar. But they all have, at least via inheritance, Islam in common. Similarly, Tamerlan's wife is a ferengi, but she converted to Islam. 

Therefore from Tamerlan's and Dzokhar's perspective, Islam was the most logical team for them to fight for. Thus, anything done by Americans to Iraqis or Afghans was an attack on their team, the Muslims of the world, thus justifying, in their own minds, attacking Americans.