If Adam and Eve had not eaten the forbidden fruit, would we all now be living in heaven, if so.........?

If God "knew" that he was going to banish Adam and Eve to earth, then the whole idea of heaven and hell would fail (assuming that we couldn't sin in heaven) but then, the fact that Adam and Eve did sin shows that we can sin even in heaven, so how is it that those of us who will go to heaven will not be able to sin? Why do they say you can't sin in heaven?