Kill mom, dad and brother leaves you being the sole heir to the family finances. They figured they would go in, do the deed,burn the house then sit back, cry victim and collect inheritance.
Her grievance could have been with the father and mother, but with the brother as well? That strains credulity. Daughter and her husband were on a mission to commit a capital crime, no matter what justification may have been in their minds. Father's actions, it appears to me, were classic self-defense.