My sister lives in NYC too and there are companies that put up temporary walls.
I have friend who lived in a small studio apartment in Geneva, Switzerland. To separate her "bedroom" from her "living room," she had a bookcase that was open on both sides, but stacked with books and nicknacks and necessities. Maybe that would work for you.
The temporary walls are a great solution. I've been in apartments divided that way and you would never know.
Shoji screens are light weight and can be configured in portable configurations. You can set them up for the privacy but still be able to fold them up and get them out of the way if you need the bigger space for a party or something.