If the character's ethnicity isn't directly relevant to the story or its message, why does it matter? People of all kinds of heritage live just about anywhere in the world, so a character being from some country doesn't have to determine their ethnicity.

I don't know how to break this to you, but mermaids don't exist.

It's a damn movie. Cast whoever is gonna play the character best. I don't care of brothers and sisters were different or if men played women or women player men.

But I'm used to the theater were roles don't have such a large talent pool.

More so, stop casting people just for their name.