Luke and Leia’s mother was a Queen of Naboo, that is true, but on that planet the position of Queen was actually an elected position, not one that came about via bloodline.
And Amidala wasn’t even Queen when her kids were born, she was a senator of the Galactic Republic.
To be fair, I’m not sure that Carrie Fisher was entirely wrong here.
Leia was a princess of Alderaan but was adopted, so her having a sibling shouldn’t matter when it comes to the family line.