@ka-chan4 To me it doesn't seem at all like incest. They are not related in any fashion, and the manner in which she was added to the family is such that everyone involved was very clear about that lack of relation. They think of her as family and clearly love her, but also just as clearly are aware that she is
not their sister.
Like if you personally feel it's incest-y that's fine, you're free to feel how you wish, but factually it is in no way incestual. Note that incest is not bad because of subjective morality or because it feels weird, it's bad because inbreeding increases the chances of genetic defects with successive generations. Arguably, given the reputation of old royalty and nobility, a single generation of incest might well go totally unremarked upon. The point being that even if it WERE incest it would still be fairly tame in regards to the setting, despite how it might offend our more modern sensibilities. Without any knowledge of how genetics works there was no reason to believe inbreeding was a bad thing, at least until the great-grandchildren started showing up with increasingly prevalent cases of clubbed feet and cleft lips.