Warning: hint of incest.
She doesn't belong. She knows she doesn't belong. They know she doesn't belong. They must know. They knew when they named her that she was different from the others. She isn't a part of this family. She belongs somewhere else, where there are others like her.
When she meets Lily Evans, she knows she belongs with her. They are two of a kind, and every part of her body recognises the other girl as a sister.
Sisters don't do this, Lily murmurs when they're in bed together.
Don't they? Narcissa says in surprise. She doesn't say any more after that, but she knows that what Lily does with her is the same thing Bellatrix and Andromeda do together, even though she's never been allowed to join them. It doesn't matter now, anyway. Lily is her sister now.
When she finds Lily in bed with James Potter, she stumbles away blindly. She knows then that her moment of belonging is over, but she can't cry. She sits quietly in the dark instead.