dr. michael davidson was shot dead by stephen pasceri at brigham and women 's hospital in boston in january .
pasceri 's sister said he blamed the doctor for his mother 's recent death .
married pasceri took his own life after shooting davidson at heart center .
davidson 's wife , plastic surgeon dr terri halperin , was seven months pregnant at the time with their fourth child .
halperin delivered daughter mikaela jane davidson april 4 , less than three months after husband 's slaying .