Responses
The part where Jeremy and Cassis' sister (the heroine of the novel) are together is just a scene from the novel itself --- it's what should have happened if the plot progressed as per written in the novel, and Cassis wasn't given to Roxanna.
So I guess, assuming that Cassis managed to escape, the heroine will not harbor any violent feelings to massacre the whole Agriche family. So from here on out, the novel should be branching away from the original plotline.
Ohhh I see, ty
Dion figured out everything about Roxanna's plan, so he is threatening to scour the Black Forest where Cassis escaped to and bring back his corpse to her.
Roxanna knows his aim is to tie her down and make her "his", in a sense that he wants to use her as a tool for his disposal --- does Dion harbor romantic feelings for her? Maybe, maybe not. BUT. He is confirmed to have some sort of obsession for her. Their family is quite f-ed up in the head after all (an understatement).
In exchange for Dion's silence that Roxanna let Cassis escape, I think she handed herself over to Dion -- making them accomplices of some kind. I think that's what the "alliance" is referring to.