Or are they? They each stand to inherit millions. They were never a happy family, thanks to their vindictive father and neglectful mother, but perhaps one of the siblings is more disturbed than anyone knew. Did someone snap after that dreadful evening? Or did another person appear later that night with the worst of intentions? That must be what happened. After all, if one of the family were capable of something as gruesome as this, you'd know.
Wouldn't you?