At no point in this novel does Mrs. Ferras seem devoid of these type of ideas or seditments. I do think Robert was hoping mightily that it would end up this way and doing all he could to ensure it. But personally, I think that even if Robert had been a decent person, their mother would have done the exact same thing. After all, she's the woman who whined yearly about paying a retirement to some trusted and faithful servants and made Lucy the star attraction to get back at Elinor.