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 .