I'm sorry, aside from the obvious, another SA, some parts of this chapter were so (un)intentionally hilarious to me - Charles-Henri thinking that the ghost of his teacher sent him a naked woman as a test and then giving him an "okay" to proceed. That WAS funny, it's just how my brain is wired.
On the other hand, it gave me a bit of Oyasumi Punpun, a bit which, fortunately, here is less visible, as Charles-Henri SEEMS to actually be enjoying this. Nontheless, there are some very complicated relationships forming right here, right now.
So, Jeanne couldn't have been all normal, since she was like a groupie for Charles-Henri's "concerts", admiring "how cute he is and how gloriously he slays". Yes, those are metaphors, if you're a dummy. I don't know yet the origin of her obsession, but if we add this to the fact that she was having a thing with that gendarme AND to the fact that she's about to be king's woman, she might just be... well, a slut. Unholy, not uncommon.
I wonder about Marie-Joseph's reaction though. Is she embarassed due to the fact itself? Is she disgusted, because of what Griffin did to her, thinking that all sexual acts are terrifying? Is she also disgusted, but because Jeanne seems to be r*ping her brother, as Griffin did to her? Those are the most normal options, one could even say "not that disturbing" (aside from those stances source, the source IS creepy).
Now... is Marie-Joseph JEALOUS of her brother instead? Better option - she's jealous, because he's the only one who's been there for her and somebody might take him away.
WORSE option - she's jealous in a romantic, twisted way, due to the same reasons. Since the whole family is so toxic, dysfunctional and demented, I wouldn't be surprised to learn that.