Of course it's "natural" the Janssens should come and stay at the home of their son and his partner.
But I doubt it very seriously.
The theory that Isabelle and the Richmonds sent Amaury to prevent a marriage between Janssen and Jarvis gains credibility in my mind because we know, per
@Clara Burnett, I believe, that when Nick Larkin's Marie showed up, Isabelle and Stephanie discussed at the dinner table their fears that she would take over the chateau if she and Nick married. So this fear of unwanted partners bogarting the highly desirable property is deep in Isabelle.
So assuming it is true that Amaury is there as the anti-Pheeph, as well as to take over as chatelain so Jarvis can travel more, is the strategic idea du jour. To me it explains the very curious coincidence that the Janssens and the Richmonds are vacationing at the shittoo at the same time.
At first I was worried they were all there -- Richmonds in loco parentis -- to preside over a Janssen/Jarvis wedding, but I think that's unlikely.
I think it's more likely the Richmonds are there to prevent the Janssens from stealing Isabelle's family silver, and to inspect Pheep's parents for Isabelle.