If Sisily's actions on the night of her father's death, and her subsequent flight, simplified matters to the extent of deepening the assumption of murder into a practical certainty, they added to the complexity of the case by giving it the appearance of a carefully planned crime in which Thalassa seemed to be deeply involved.
The insistent necessity of motive which should explain the events of that night with apt presumptions, threw Barrant back on the suggestion, made by Austin Turold, that it was really Sisily whom Mrs.Pendleton had detected looking through the door of the downstairs room when the other members of the family were assembled within listening to Robert Turold.

Barrant told himself that Mrs.Pendleton's suspicion of Thalassa rested on nothing more substantial than feminine prejudice, an unreasoning impulse of dislike which would leave few men alive if it always carried capital punishment in its train.
The substitution of Sisily for Thalassa provided a convincing motive for murder.

The overheard revelation of her mother's shame and her own precarious condition in the world when she might reasonably have been counting on becoming an heiress of note, were sufficient to account for the nocturnal return and an effort to entreat justice or compel silence--the alternatives depended on the type of girl.

From what Mrs.
Pendleton had told him of Sisily and her love for her mother--poor Mrs.
Pendleton had insisted, all unwittingly, very strongly on that--Barrant had pictured her as a brooding yet passionate type of girl who might have committed the murder in a sudden frenzy of determination to prevent her father making public the unhappy secret of her mother's life.

That was an act by no means inconsistent with the temperament of a strongwilled and lonely girl, whose stormy passions had been wrought to the breaking-point by disclosures made on the very day that her loved mother had been buried in a nameless grave.