If you identify as Ash Ketchum, you’re not saying “I should have been born a human child in the Kanto region”,
First of all, there are lots of kin who experience being kin in exactly that way. Wolfkin who are saying they should have been born as wolves, etc. I have met fictionkin who fit that framework as well.
you’re saying you’re a particular person with a particular personality and set of experiences.
People are kin for one of two reasons: they feel they are a species other than human (otherkin), OR they feel they are a specific person other than the specific person they are on this earth. (human fictionkin). Or a mix of the two.
Royalty isn’t a state of being nonhuman, or a species at all, so it isn’t being otherkin.
Generic ‘royalty’ isn’t a specific persona with a set of memories/feelings/experiences, so it isn’t being human fictionkin either.
You can be Sailor Moon, (who is a princess) but that makes you fictionkin, and sailor moon kin, not princess kin.
You can be Cleopatra, but that makes you reincarnated, not princesskin.
You can be Queen Elizabeth, but that makes you factkin, and queen elizabeth kin, not princesskin.
You can be a random princess with a name and lands and history, etc, that isn’t from history or any fiction that you know of, but that makes you OC fictionkin, and the name of that person, not princesskin.
You can be a person in this life who feels like they should be treated like a princess, but that makes you a spoiled brat.
Likewise, a royalkin probably isn’t identifying with royalty as a physical organism, but how royalty exists and the intense feeling that their existence should match.
There is not metric or expectation for ‘being royalty’ though. Any action that is undertaken by someone royal is royal.
Is acting as royalty about acting with justice and humility? or is it about ordering people around and being fed the best food?
Royalty is a job, not an identity.