D.H.
Tell her that Mommy needs some sleep to help the baby to be happy when she is born. Tell her that you love her and want her to be happy about staying in her room. If she can stay in her bed/ room until it gets light outside, she will get a special privilege (go to the park, etc.) And the baby will be happier when she is born (if she's not happy when she's born, then you tell her sister that she - the baby - can't help being sick - it's not her fault). Good luck!