I have a theory, just based on what has happened to me. I think Katie might be right about the non kin who change their vibration because of their daily/personal connection with an Otherkin.
I don't think anything could make me Otherkin if I'm not, but I think an understanding vibration, a co-existing vibration is possible.
I think when non kin (I find I prefer the term non kin to the word mundane.) don't know anything about Otherkin then we don't have to try and understand, but if someone in your life trust you enough to tell you their essence is kin, then, for some who really want to connect, to share their lives and their hearts with someone who is kin, then it is possible to move yourself closer to that persons vibrational frequency in order for connection to take place.
I also think that having a person who is kin share their lives, their feelings, their memories, their hopes and their dreams with you brings to you a new realisation of who they are, who you are, and who together you will become.
It doesn't make you kin, but it brings you as close to it as a non kin will ever get.
I know I was stunned when Jay told me about himself, and yet it never occured to me to think that he was either mistaken or being untruthful. I trusted him, and a lot of the things he was telling me made the person I had always known make so much more sense. It was almost as if my brain cleared like the sun through the clouds on a rainy day, and for the first time I really saw him. For the first time everything about him really made sense.
From that day everything about me changed, except the way I felt about him. That hasn't altered since the day I met him, standing in a school playground, looking just like me, tall, skinny, lost and looking for a friend. (I hated starting a new school.)
The only thing Im sorry about is that it took so long to learn this about him, I feel like if I'd known about him sooner I could have been there for him more. But he didn't really know, so how could he tell me?

He's been my best friend for so many years now that a little thing like finding out that inside he's a kind of burning bird creature, thirty five feet high, with a twenty foot plus wing span is just one of life's little details.

Love Ya Jay.

Kes.