The war is over, Voldemort is gone, but status quo and a life's worth of beliefs don't die with the man who helped enforce them. They live on with the rest of her, inside of her body, inside of her mind. Her beliefs are a part of her and, though she has lost the war, she has not lost herself.
Narcissa never admitted that she was wrong. Narcissa will never admit that she was wrong.
The war taught her that blood purity was important, that it was something to fight for, but it also taught her that family trumps duty. A life is not burned out as easily as a face on a tapestry.
She invites her sister over for tea. She only has the one sister left and there is no use denying her any longer, but Narcissa still can't help cringing when she sees that Andromeda has brought Teddy. It is an undignified name for an undignified boy and he tracks mud on her rugs like the mud in his veins. A half-blood without the proper allegiances is just as bad as a mudblood. She keeps her thoughts to herself.
They are not young girls anymore, Narcissa still the younger by two years, but the gap looks so much larger. Andromeda is a woman of nearly fifty, but the lines in her face show her losses. A husband, a daughter, a son.
Some might consider a grandchild a blessing, but Narcissa knows he's just as much a reminder of the toll the war has taken on her sister, of things that cannot be replaced.
She invites her sister over for tea and she allows the filthy child to play on the couch (she will burn it later, or else the house elves will do it for her), and she does it all with a smile. She embraces her sister and kisses her cheek and seats her in the finest chair. She introduces her son, her husband, and this time when she smiles she smiles smugly.
Narcissa has lost the war, but she has not lost her family. This war is over, but there will be more. In the future, Narcissa suspects that Andromeda will be wiser.