From Six to Sovereignty: The Girl Who Ruled - Chapter 12

Technically, if we use the British royal family’s rule for succession. Even before women were given fair rights for it. Our FL will be the next successor; this was also how we got queens such as Elizabeth and etc. The crown or in this situation the head of the family will favor the offspring of the successor instead of the sibling of the successor and even if its a woman, they will be given the title. Its just if the FL had a male sibling no matter younger or older, that’s when they lose their right for succession. They only have female successors if there is no longer any males to favor over them.

TL;DR: In the eyes of the British Royalty’s rules for succession, our FL would have been the successor instead of the uncle.