Its such a big change for her with a new baby there, i would be reluctant to force separation at this point. I think having the baby in with you for a while and the 6 yo on the same level is a good solution for the moment, and look at moving her upstairs on her own when the dust has settled a bit. Could you have both of them upstairs and you guys downstairs? This might be a bit more appealing to her, knowing she is not upstairs on her own and its a bit of a novelty?