When Estela reveals her true heritage the daughter of minor nobility in service to Toulouse which is antagonistic to her current patron it throws a wrench into The Plan Emerganda and Alienor have to see her securely married because of family politics. Emerganda shows her Chess Master creds by making up bullshit about how her father's immediate liege lord gave her premission to chose Estela's husband. The awesome part is that everyone will know that it's bullshit but won't matter because the guy's dead and so no one can contradict her.
Emerganda appropriates the Big Bad's Evil Plan and twists it to resolve recent ethnic unrest. By claiming Arnaud's murderers were jewish she can give her Christian subjects the jewish scapegoat they wanted without killing any real jews.