I think Elsa and Ana get added to that list 
There is a whole process that a character appears to go through before they are designated a "Disney Princess".
It is very arbitrary and is more tied to marketing and the ability of the company to make money from the designation.
Over the years some have been stripped of the title, others have been clumped together but never officially acknowledged.
Then there are those that hold the title but for some very weak reasoning.
Here are the basic "RULES TO BE A DISNEY PRINCESS" that are taken from the official wiki and inferred from the exclusion of some actual Princesses - which just goes to show how arbitrary they are.
Rule 1 =
You must be human or nearly human - This is why all the anthropomorphic Princesses are not included also Tinker Bell has been removed from the list. Ariel just makes it on the list as nearly human.
Rule 2 =
You must be a primary character not in a squeal - This is why Ariel is a DP but her daughter Princess Melody is not.
Rule 3 =
You must not be "too young" or "too old" - The minimum age now appears to be Ariel and the maximum is Aurora - this has excluded several actual Princesses from the list and is the weakest rule of the lot. It may also exclude Elsa from running (see rule 5) - this is a range somewhere between 16+ and about 21 - This is probably why Eilonwy, Kida and Vanellope von Schweetz are not Princesses (Kida may fail due to Rule 5) - There is also the rumor that Eilonwy is Aurora's older sister thus making her oddly too young in her own movie and simultaneously too old in her sister's movie as Aurora is roughly 21 at the end of her film making Eilonwy older yet.
Rule 4 =
Your G-rated film must have been a financial success - This may have been how Mulan made it into the ranks - she is the exception to rule 6 and how no PG-rated Princess has made it on the list.
Rule 5 =
You can not have been made a Queen during your film - This blurs the lines between what you are or can be called in subsequent marketing and could explain how some true Princesses have been excluded, like Elsa as she is a Queen.
Rule 6 =
You must be able in some manner to claim the title of Princess or its equivalent - Thus Jasmine as Sultan's daughter or Pocahontas as Chieftain's daughter are allowed on the list.
Rule 7 =
Your character must fit the Disney paradigm of what they say a Princess is - so if you are too dark, of questionable morals, a bit too independent, etc. then you are excluded. The limit to this is Merida who just makes it on the list however it excludes characters such as Kida or Megara.
Rule 8 =
Your character is only added if a Disney property at the time of your film - there are no retroactive additions to characters outside the Disney-verse at the time of the film - thus Princess Leia is not, any Marvel character is not, any independent Pixar, etc. is not. You have to have been Disney at the time of your film introduction.
There are entire websites devoted to trying to get some characters on or off the official list but it is all driven by money and marketing so it is not really based off a logical definition of "Princess".