Narcissa Malfoy has lots of memories of Bellatrix, yet she only acknowledges a few of them. They're the ones of her sister; Bella as she'd call her then, back when she'd idolised her.
Bella stubbornly jutting out her chin when being reprimanded for rough sports, Bella waving her hands around animatedly as she told stories and Narcissa listens, captivated. Bella driving all their tutors crazy because no one could make her do anything she didn't want to do.
Narcissa had cried when Bella was leaving for Hogwarts and couldn't be placated until her sister had promised multiple times to send her lots of letters. When Bella came home, she'd pulled Narcissa aside, "Look Cissy! I can show you lots of spells", and even though she had absolutely no patience of any kind she'd promised to teach Narcissa too.
Sometimes, Narcissa wishes that they'd never grown up.
She remembers when Bellatrix had proudly shown her sisters the dark mark; they'd started to grow apart then, but Bellatrix was proud to show off her achievement. Andy had been oddly quiet that day but Narcissa had gushed over her sister and oh how mature she was. She'd listened to Bellatrix boast with the utmost admiration and now that thought makes her skin crawl.
When the first wizarding war started, she'd rarely seen her sister at all. She'd known of her sister's ruthlessness then, had finally taken off her rose-coloured glasses, and yet the news that her sister, her sister, had tortured two people to insanity came as a tremendous shock. Narcissa could understand having to maim people or even killing people, for there were casualties in a war. But to torture them?
Her sister had grown up and discarded all sense of humanity.
When Lucius was declared not-guilty by the Wizengamot (imperio-ed, they'd claimed though they both knew that it was a lie), she was overjoyed and so relieved. She'd felt a smidge of guilt when Bellatrix was sent to Azkaban, but it had been her own fault for so haughtily agreeing to the crime, to say nothing about how she talked about Voldemort. (Though what she'd expected her sister to do she didn't know; after all, the crime was clear cut.) She'd never visited Bellatrix and had never even tried to.
After the war she'd gained a new-found appreciation for every day and every little blessing. She fussed after her baby, her little Draco and laughed with her husband. She'd never really let go of the idea of a happily ever after. She'd thought that she'd got her happily ever after, that all was well, and was caught unawares by the second wizarding war.
She realises now that it hadn't caught her unaware; she'd merely pretended to not see how the wind was blowing. As if ignoring it would make it disappear.
The first time that Narcissa saw her sister after Azkaban she'd nearly tumbled to the floor in shock. Bellatrix was gaunt, her shoulder blades jutted out and her cheeks were hollowed. Her eyes were sunken, and her hair was such a tangled mess. Narcissa could see the wrinkles across her sister's forehead and the veins clearly visible through her hand. She resembled nothing of her sister at all and Narcissa nearly started crying then.
There was still something of her sister there though, and she nursed her back to health. She was never quite her Bella again though. Maybe she never forgave her for not visiting. Maybe. They never really talked to each other. Narcissa couldn't help feeling pity for her sister, how could everything have gone so wrong, and Bellatrix loathed it.
When Draco was assigned to kill Dumbledore, Bellatrix raved about it. That was the first time that Narcissa felt pure hatred towards her sister. Never had she hated her sister as she'd hated her at that time and Bellatrix had actually flinched.
When Bellatrix fell at the Battle of Hogwarts, Narcissa was the one who buried her. She tells herself that it was out of familial connection and not out of any affection, for this Bellatrix wasn't her Bella. She didn't cry, didn't shed even a single tear for her sister that day.
Yet that night she woke up gasping, tears streaming down the sides of her face. Her heart ached when she remembered that her sister was gone, her Bella was gone. She'd lashed out against Lucius when he'd asked her what she'd dreamt about. She loathes Bellatrix for appearing in her dream, even though it was no fault of hers. She loathes herself for crying, for who cries over a torturer? She hates herself for hurting over her sister when Bellatrix had felt no qualms over Draco's life being in danger. Narcissa briefly wonders how Andromeda feels knowing that her own sister murdered her daughter.
Narcissa tries very hard not to think about Bellatrix but it deems to be impossible. For nights on end she sees her sister and wakes up shaking, Bellatrix's voice ringing in her head. Sometimes she sees her sister as she was before and Andy too like they'd never grown up. Other times she sees Bellatrix after Azkaban, slitting Draco's throat with that knife of hers all while looking at her with a manic grin. She isn't sure which is worse; waking up missing how things were and knowing she'd never get it back or waking up screaming and trembling and scared of her own sister.
Later, much later, Narcissa confides in Lucius and he holds her close. She doesn't think he quite understands though.