The Disney Princess brand has very little to do with who's actually a princess in their own canon and who isn't. It's a franchise created to sell merchandise.
Disney execs decide which girls get "coronated" into the official Disney Princess lineup, and that's that. If you're in the lineup, you're an official Disney Princess. If you're not, you're not a Disney Princess even if you're a princess in a Disney movie, see Eilonwy, Kida etc.
Technically Elsa, Anna and Moana aren't Disney Princesses either, but Disney seems to treat them like honorary prospective members so I'd guess Moana will officially be included within a couple of years, and Elsa and Anna once the Frozen franchise has run sufficiently out of steam.