In looking at the avatars next to your sister’s two profiles, is one of them the correct animated avatar that she actually chose (Mickey, Minnie, Belle, Buzz, etc.) and the other avatar a generic blue silhouette of Mickey’s head?
I have found that whenever I mistakenly have two matching profiles in my Family and Friends list it’s the generic blue Mickey that is the impostor. I remove the false profile from my list and all is well. Also, I’m assuming only one of your sister’s profiles has theme park tickets assigned to them? If yes, when you begin making FPs you will quickly be able to tell which profile to use because the other won’t have tickets and therefore will not be able to book FPs. Hope this helps.