This was such an original story. I never read a story where Narcissa was actually Nymphadora's mom. I really loved this. And I have to admit that I would love to know what happens.
I mean, in this universe, Nymphadora would be Harry's half-sister, and I wonder how Narcissa would feel upon meeting Nymphadora for the first time, maybe in the height of the second war.
I loved the scene between James and Narcissa in the beginning. It was intense and the image you evoked was beautiful.
I think how they handled the baby was actually very realistic. Most authors might have them playing happy family, but considering the Black family, that's not believable even if people enjoy happy endings.
I would love to possibly write a sequel or continuation of it, if you don't want to. Would you be okay with that?

WHOA! Interesting and well written one shot.