I believe Tamerlan Tsarnaev brainwashed his younger brother because young people can be impressionable. This is especially true when it's a situation where a younger sibling looks up to an older one. This can be further compounded depending on the relationship they had while growing up. When an older sibling does something that seems important, admirable or good then it is not surprising that the younger sibling might follow in their footsteps.
I do agree that his older brother has influenced and ultimately led to his younger brother following suit. This is true of most older brothers and it led to a terrible tragedy. His brother is seven years older than him so I am quite sure this act was planned from the beginning. It has been said that Tsarnaev followed his brother around like a puppy.
The nineteen year old was in his right mind. He confessed to his part and knew what he was doing. I know he looked up to his brother and felt he was right. I think he was heavily influenced by this older and wiser man. In my opinion, had his brother not been there, the nineteen year old may have lead a better life. I don't think he was brainwashed; I believe he idolized his brother. The younger man seemed to fit into our society, but he listened to the wrong people.
The young brother looked up to Tamerlan. I do not think it was a matter of brainwashing. It's more a matter of love and wanting his brother's acceptance. The fact that Tamerlan had poor views on America certainly influenced the kid. Whether or not the entire family shares the same opinions as the boys did could have also played a factor.