Family is everything to the Blacks. They are taught so before they even know what the words mean.
Narcissa shares Bellatrix's perspective when their sister ups and leaves them. For mudblood filth, no less. After all, aren't they better? Shouldn't they have been enough?
Why weren't they?
The two remaining sisters fury grows when their cousin joins the ranks of blood traitors. Everyone had such high hopes for Sirius, although they knew it was an inevitable downhill spiral as soon as the Sorting Hat said 'Gryffindor' and he was stolen away from the watchful gaze of his family. They take Regulus into their ranks and the three remaining Black children now adults huddle together close against the approaching storm. But discord screams within them.
It is not long before their family is all but destroyed. Andromeda removed long ago, Sirius and Bellatrix dwindling in Azkaban. Regulus in a cold grave somewhere, their hope for a legacy destroyed, the shattering of his youth a reminder to them all. Narcissa becomes more indifferent and more cold to those outside her circles, burning with love for the only little star and legacy she has left, her Draco.
When Sirius escapes and Voldemort's rises again, they are moved into place, little worn, little black chess pieces fighting against each other. The sting of betrayal rises in Bellatrix as she propels her cousin back and into the veil beyond, never forgetting the look of surprise on his face, that it was his time so soon, that it was her to do it, although it had a resounding symmetry. Alone, in the darkness, away from duty and watchful eyes, she does not feel guilt but a pang of regret that it had to be like this. It only hardens her more.
Later, when all is said and done, the Dark Lord has been destroyed, the Wizarding World left dazzled, they flee. Their husbands dead, Draco gone, they run and run, a dark and fair figure, each of flawed beauty, hand in hand, until the encroaching tendrils of the veil pass over them, like a night without stars.
There amongst the whispers they reach out to their similarly fallen kin and never let go. What lay on the other side does not matter here, they are victims to circumstance no longer. Instead they wait patiently for the first who left them, until they are finally home.