Not a Happy Family

In the quiet, wealthy enclave of Brecken Hill, an older couple is brutally murdered hours after a tense Easter dinner with their three adult children. Who, of course, are devastated. Or are they ? Each stands to inherit millions. They were never a happy family, thanks to their vindictive father and neglectful mother, but perhaps one of them 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 ?