Dollars and Sense
At the moment an investigator decides the motive for a murder is money and/or love, the next question has to be: Who had the opportunity? Whenever an entire family is taken out, any surviving members of that family are, quite obviously, carefully watched and interrogated, as Dana had been.
Dana had motive.
More like seven million motives, which was the sum of money his father's estate was said to be worth.
Dana had even admitted to writing the family's net worth statement recently—which told investigators that he knew full well what his future inheritance would be.
He also knew that he had to wait to receive that money and—damn-it-all!—split it with his sister.