@Mira995 ,
@Nixnaw and
@Oinkment
Like huapollon said this takes away the option the other marquisse could have done with using her mother. Because Estelle doesnt care about her mother (her mother deserves this) she cant use her as a hostage behind the scenes. If the Castellio claim that Estelles mother is dead, then it would turn into a fact. I need to say this is a splendid way of destroying the enemies trump card. Because the mother doesnt openly showed Estelle to the outside and did keep "secret" about her there shouldnt be evidence either.
Therefore IF they claim that she is Estelles mother and call the Castellio liars, it will turn into a gigantic political mess. No one would want that, not even the Royal family, which would be the only ones who could force the Castellio to admit it. But......... even if this happens, the Castellio dont care about it. They already have a somewhat scary/bad image and therefore it cant get worse and no one can benefit from it. Thats why it is unlikely that any royal with even half a brain would take all the bad points without any benefits from it.
PS: If they send assassins it could be resolved too, but if it fails or something like that it could end in trouble too. Both methods are possible, but Estelle her move minimizes the potential backlash for the Castellio. (Which they normally wont care.)
In my opinion would one of the best options be first declaring that with the mother and then using the assassins for her mother.
That way she cant be used in any way. (IF the assassins didnt get caught, but that is a gamble because I do believe that the other side protects Estelles mother against that.)