If you're a multiple...well, that's what the "other" option is for.

You may determine your birth order out of just "whole" siblings, you may include "halves" and "steps" if you like, you may consider just those siblings who you grew up around regardless of how they related to you, adoptees may consider either their birth families or their adopted families, what-fucking-ever. However, if yours is a situation that needs qualifying, please explain so in your reply.

Me, I'm an only. Simple.

On edit: Fascism removed from determination of birth order request. Thanks, Gormy.