It all comes to boiling point when oldest daughter Julia is killed and her sister Ada wounded in what is initially thought to be a burglary gone wrong. It soon becomes obvious it was no such thing and the siblings do not hesitate to air their grievances against each other to DA John Markham, Philo Vance, and the usual company. In fact, when it comes to suspects, a third daughter, Sibella, declares flat out "If you're looking for possibilities you have them galore. There's no one under this ancestral roof who couldn't qualify."
The household consists of bedridden Mrs Greene, the four children left (Ada, Chester, Sibella, and Rex), German cook Gertrude Mannheim, given to going about muttering to herself, the maids Hemming, convinced God is smiting down the sinful Greenes, and Barton, who rightly reckons "There's something awful funny going on here", plus elderly butler Sproot who reads Martial, although only a snob would find that suspicious. Then there's Doctor Arthur Von Blon, who visits more often that caring for Mrs Greene would seem to necessitate.
Soon there is another death in the mansion....
My verdict: While I would not go so far as to observe that lying beneath the Greene case are what Van Dine in Lovecraftian mode describes as "obscure fetid chambers of the human soul. Black hatreds, unnatural desires, hideous impulses, obscene ambitions", the set-up is bizarre enough to make readers pay close attention to the details and the map of the house, and therefore one or two clues will probably be identifiable fairly easily. One vital to the solution is not presented in quite fair fashion unless you stretch your definition of fair, in which case I will begrudgingly agree that, well, perhaps it was. I wavered between three possible culprits and one of them was the right one -- but in all fairness, it was the last person I began to suspect. A dark novel.