The other world in Coraline is a strange version of that reality that Coraline knows well. It has the same people, the same house, and the same garden; but something's just not quite right. This world seems so much like reality precisely because it was custom made to be that way. Luckily, Coraline is pretty discerning: she knows that if something seems like reality, but just the best version of it, it's probably too good to be true. This knowledge allows her to see what a sham, or fake, the other world really is. But there's something more to think about: if the other world really exists, isn't a reality in itself? Is it the other mother's reality? Or reality for the three ghost children? What do you think?
The other world was a reality of its own.
Coraline's other mother got lazy when she just created Coraline's house and didn't create the rest of the world.