I'd love to hear what some of the sharp fiduciaries around here have to say. I can't add two and two.
As Fanny was the first to point out, Isabelle has more than a little of the chiselling cheap French bastard of Jean de Florette/Manon des Sources. Added to her natural proclivities, post-WWII France could well have been as deprived as post-WWII England. I don't think she's quite old enough to have been out and about too much during the war, but that was a big fat deal.
This relationship is defining. Me and my MD degree in psychiatry
count that Derek's many inappropriately generous gifts to a daughter, the curious painting he made of Isabelle calculating and playing games while pregnant, may have gone a long way into creating a narcissist.
Isabelle's whoring, basically, for free electrician work, which required turning out the (gay) caretaker couple and the ensuing charges of homophobia filed, I believe, with Workaway, is the hallmark of her viciousness and chiselling.
And Stephanie believes in this, while being frightened of her disapproval. Perhaps she thinks "Mummy"'s draconian accounting will make everything okay in the end, as long as she keeps up the chiselling and feeds her employees/guests the stingiest possible eff-you meals and does not heat the shittoo. I think those are iconic cheap French bastard values Jarvis treasures.
I also wouldn't put it past her to be cheered by seeing Isabelle take the fall for any and all tax evasion.