~의 위에 = above, 위에 = on

I have the same question.

I think (correct me if I'm wrong!) 우리의 means "us" if it isn't followed by a noun.

The marker 은 puts the sentence focus on the sky so that it's more like "As for the sky, it's above us." It sort of answers "What about the sky?". In the sentence "Above us is the sky" (I think: 우리의 위에는 하늘있어요) the focus is on "above us" as though you've been asked "What's above us?"

the the sky is not the subject of the sentence