Jim Dale, on the audio tapes, gave her an accent.
That never did make sense to me. They're sisters.

Does anyone know how old they are?
If they have a huge age gap and the family moved at the
right time, one could have an accent and not the other.

Come to think of it, Narcissa doesn't talk much.
I don't think we hear her at all until HBP.
I just asked my daughter, who has heard the tape a bunch
of times, and she says that Narcissa has a very slight accent.

By the way, their maiden name is Black.