The mother-in-law of one of one of the Boston bombers has taken to the stand to tell how her daughter was converted to Islam by her husband, as lawyers try to argue the other brother out of being put to death. Judith Russell, whose daughter Katherine married Tamerlan Tsarnaev, one of the two mass murderers who set of bombs at the 2013 Boston marathon, spoke at a court in the city today. Her words came as jurors decide whether Tamerlan's brother, Dzhokhar Tsarnaev, 21, should be put to death after being convicted of a host of crimes in connection with the massacre. Scroll down for video . On the stand: Judith Russell, pictured right in a 2013 file photo, took to the stand in the trial of Boston bomber Dzhokhar Tsarnaev. She told how her daughter Katherine, left, converted to Islam after meeting Dzhokhar's brother Tamerlan . Dzhokhar Tsarnaev, pictured three months after his arrest for the Boston bombing, flipping off a camera in his jail cell. His defense team argued on Monday he is a 'good boy' and should be spared the death penalty for killing three people and injuring several hundred in the 2013 terrorist attacks . Tamerlan himself was killed in a police firefight a few days after the bombing, which he masterminded. Change: Katherine Russell is pictured in a yearbook photo, before she converted to Islam . Defense lawyers for Dzhokhar have made the argument that he should not be killed because he was in some capacity forced into the killings by his more persuasive elder sibling. Russell was brought before the jury as a defense witness - and told how Tamerlan met her daughter and got her pregnant before gradually becoming more radical in her views. Russell told jurors she drifted from her daughter as a result of the relationship - and that at some point was shocked to find her wearing the veil and a new convert to her husband's faith. Though Russell admitted that Katherine seemed to take on the religion of her own volition - and still upholds it - her testimony indicated that he was the cause and she considered the move a sacrifice for him. At one point she revealed that Tamerlan, who was originally unemployed and unmotivated, became so zealous in his faith that he tried to force literature on her, too, and encourage her to convert. She told jurors that she has an Islam For Dummies book on her shelf at home, but couldn't remember who gave it to her. She said she had hardly met Dzhokhar or his family, but that she remembered attending a baby shower which the brothers' very religious mother attended. Dzhokhar Tsarnaev, an ethnic Chechen, was . found guilty earlier this month of killing three people and injuring 264 as well as fatally shooting a police officer. David Bruck delivered the defense's opening statement in the penalty phase of Tsarnaev's trial, claiming that no punishment Tsarnaev can be given is equal to the suffering of the victims. Influencer? Defense lawyers for Tsarnaev (left) have said that his brother Tamerlan (right) drove his brother to help him with the 2013 killings . 'There is no evening the scales,' Bruck said. 'There is no point in trying to hurt him as he hurt because it can't be done.' During the initial stage of his trial, prosecutors portrayed him as an extremist who wanted to 'punish America' for military campaigns in Muslim-dominated countries. The teen followed a blueprint from al Qaeda's Inspire magazine to build the homemade pressure-cooker bombs that ripped through crowds of spectators,volunteers and athletes on April 15, 2013. Bruck urged the jury to sentence the defendant to life in prison without the possibility of ever being released for his crimes. A chilling photo was revealed last week at the Boston bombing trial showing the aftermath of the blasts on Boylston Street . 'His legal case will be over for good, and no martyrdom, just years and years of punishment,' the lawyer said. 'All the while, society is protected.' Bruck told the court that Tsarnaev would be sent to a supermax prison in the Colorado mountains, adding that he would have no privacy, be under a video camera 24 hours a day. He would be allowed no interviews, there would be no autobiography and no messages relayed, the attorney added. Bruck focused heavily on Tsarnaev's  brother, Tamerlan, who died after a gunfight with police four days after the bombing, saying he led the plot and provided the 'fuel' to drive the plan. He said Tamerlan was 'consumed by jihad' and had 'power' over Dzhokhar, who admired his older brother. Bruck contrasted Tamerlan with Dzhokhar, saying Tamerlan was loud and aggressive, got into fights, failed at everything he did and never held a steady job, while Dzhokhar was a good student in high school, was loved by his teachers there, had many friends and never got in a fight. 'He was a good kid,' the lawyer said. But he said Tsarnaev started going downhill in college. One of the first defense witnesses to testify in the penalty phase described an incident at a mosque when  Tamerlan became angry and interrupted prayer services. Imam Loay Assaf testified on Monday that he got very angry during one service in 2013 when Assaf praised Martin Luther King Jr and compared him to the Prophet Mohammad. Assaf says Tamerlan interrupted the service, shouting at him and calling him a 'hypocrite'. Prosecutors have portrayed Tsarnaev as an unrepentant killer who gave the finger to the security camera in his jail cell three months after his arrest. The startling photo and video were shown to the jury last week. Bruck downplayed the gesture, saying Tsarnaev was 'acting like an immature 19-year-old'. Defense attorney, Judith Clarke, had opened the trial . with the blunt admission 'it was him,' Tsarnaev, who committed . all the crimes he was charged with. But she closed the guilt . phase by arguing that, when it came to planning the attack and . building the bombs, 'Tamerlan did that.' Defense attorneys Timothy Watkins, left, and David Bruck arrive at federal court on Monday during the penalty phase of the federal trial of Dzhokhar Tsarnaev, who was convicted of the Boston Marathon bombings that killed three and injured 260 people in April 2013 . In this courtroom sketch, bombing survivor Marc Fucarile, right,  was depicted on the witness stand on Friday during the penalty phase where he stared down Tsarnaev. Fucarile said he has had more than 60 surgeries, and it is still unclear whether his left leg can be saved . In a sharp contrast to the last phase of the trial, when the . defense took just two days to call four witnesses, the court has . scheduled about two weeks' worth of testimony as Tsarnaev's . lawyers make the case to spare his life. Few details are known about who the defense lawyers will call to . testify as all witness lists in this trial have been filed under . seal. On Thursday, local media reported that some members of . Tsarnaev's family arrived in Boston. It is not clear exactly which members of his family traveled . to Boston or if they will testify or simply observe the . proceedings. Another unanswered question is whether Tsarnaev will speak . in his own defense. He has been a quiet presence during the . trial, offering no signs of emotion as the jury heard sometimes . tearful testimony from survivors and viewed graphic, disturbing . images of the bombs' detonation and aftermath. Eight-year-old Martin Richard, Chinese exchange student Lu Lingzi, 23, . and restaurant manager Krystle Campbell, 29, died in the . bombing. The Tsarnaev brothers shot dead Massachusetts Institute . of Technology police officer Sean Collier three days later.
Dzhokhar Tsarnaev was found guilty of killing three people and injuring 264 as well as fatally shooting a police officer .
His lawyers on Monday argued for the jury to spare him the death penalty .
Called Judith Russell to the stand, who is mother-in-law of Dzhokhar's older brother Tamerlan - who was killed in police firefight after the bombing .
Told how her daughter was enthralled by Tamerlan and became Muslim .
Backs up defense claims that Dzhokhar was led by influential sibling .
Lawyers have said that Tamerlan drove his brother into the bombings .