As details emerge from the events that led to the tragic attack in Boston, it seems as though older brother, Tamerlan, was the alleged driving force behind the bombings. Younger brother, Dzhokhar, was influenced by his older brother to go along with the plan. The shock that people who knew the suspects are expressing is primarily about how well-mannered, studious Dzhokhar was able to commit such an act. As for Tamerlan, he was known as a difficult fellow, apparently even by the FBI.

This leads to the obvious question: How is possible that an older brother could have such sway and control over a younger one?
Let's begin with what we know about sibling relationships in general. Research on siblings, although not a widely studied field, seems to indicate that through a process referred to as sibling modeling younger brothers often see older brothers as the model for their own behavior. Studies show that younger siblings are much more likely than others to engage in drug abuse and delinquency if their older siblings are engaged in such illegal activities. Although sometimes younger siblings try to follow a different life path than older siblings in order to stand out, referred to in sibling research as deidentification, this usually happens with siblings who follow one another in the sibling line and are close in age to one another. However, when there are other siblings in between or when there is a large age gap between the siblings, such as in Tsarnaev brother's case, younger siblings look up to their older siblings and follow their lead.
Although it would be hard to believe that this type of modeling would make an older brother influence a younger brother to such an extreme degree, when examining the unique situation of the Tsarnaev brothers there are additional dimensions to the individual, sibling, and family dynamics in this situation that may help shed light on younger brother Dzhokhar's transformation.
First, considering the apparent detachment and disconnect Dzhokhar may have experienced from his parents, Dzhokhar may have relied on his older brother to compensate for the lack of parental love. From the stories coming out of the family, including the estrangement between various family members, and the extremely harsh demeanor exhibited by both parents over the past few days of interviews, Dzhokhar may have experienced what is referred to by sibling researchers as a vacuum of parental support. This easily could have led Dzhokhar to overly depend on older brother Tamerlan including being excessively influenced by Tamerlans' ideology.
Second, considering the age of Dzhokhar, developmental psychologists would say that he was in the midst of developing a personal identity. This occurs when adolescents, during high school and college years, seek to find their own niche in the world. Based on what is known about identity development, it is likely that Dzhokhar did not do much seeking of his own identity and simply took on the identity of his older brother. Healthy identity formation requires a democratic and warm home environment which seems to be lacking in this situation. Additionally, identity development is more difficult for immigrant adolescents. Thus, as an immigrant, Dzhokhar simply went along with assuming older brother Tamerlan's identity and ideology.
Finally, advances in brain science suggest that the prefrontal-cortex, the decision making part of the brain, is not fully matured until the early twenties. Add to this immaturity the sibling dynamics just examined and we've got a highly impressionable, vulnerable, and easily influenced younger brother.
Once his older brother was killed, Dzhokhar was left alone, confused and leaderless. Found huddling in the fetal position in a backyard boat, he is now apparently cooperating and communicating with law enforcement, highlighting his ease in being influenced and his profound impressionability.
http://m.naij.com/news/31975.html</a>

I think for an elder brother to influence his younger brother to perpetrate violence.
The younger brother should have some elements of violent tendencies which the elder brother ignited.