It stands to reason that an instinctive reaction to being attacked with a knife would vary from person to person.
Maybe the mom's first reaction was to recoil or be frozen in terror. This could have allowed the attacker sufficient opportunity to deliver a lethal wound with the first blow. If the father and the brother reacted by fighting back, many more strikes would have been needed to be sure that they would not survive their wounds.
I think that most people,(male or female) would fight back. In real life I know I would fight, but in nightmares I sometimes find myself paralyzed in fear; unable to scream. I wake up making some pitiful moaning sound.
Maybe the mom was speaking so quietly because that was all she could manage to do, like I do

in my nightmares.
Susan