J.H.
She's your sister. Of all the people you can be blunt with, she's one of them. Just tell her that you're honored that she wants to do a baby shower for you, but you'd rather wait until the little one is here and do a meet the baby party. Then ask her if she wants help planning it, or tell her that if it gets to be too much (the planning) just to let you know!
*hugs* (And CONGRATS! on the pregnancy!)