The one who decides who gets the sword is the god, Sylph. So far the sword had always been inherited by people from the royal family, but this time the god picked a nun instead of the queen. I hope we get a proper answer as to why. Did Natalia lack some qualification? I want to know.
I don't think that's it. The exact phrasing Sara used in chapter 1 was "The real sword is here rooted in me" and not something like "I had been granted the sword" or similar. If I had to guess it's a ploy by Slyph to give Natalia someone she can rely on and keep by her side.
This is only a guess of course, but if it's correct it's definitely working.