My aunt was like with DM for years and years. It just got worse and worse- critical attacks at family gatherings, insane competitiveness over who'd host family Xmas/buy the best presents, really, really hurtful horrible stuff.
My aunt didn't want to deal with and actually really enjoyed it I think, especially as my mum got older. My aunt had always been a bit overshadowed when they were younger, both academically and physical attractiveness, and never really got over it. When younger she always tried it on with my mum's boyfriend's apparently, including making a pass at my dad one time. I didn't believe that myself until I her her flirting wholeheartedly and outrageously with my mum's ex at my mum's funeral. Which kind of tore it for me tbh. He was pretty shocled and having none of it to be fair to him, as he knew she had form.
Anyway, my mum felt pretty guilty about being more successful when they were younger, and also a few things like thumping her when they were under 5s. She tried and tried for years, and put up with cruel things like being ordered out of the house, gossiped about, sent to coventry, going round one year with a birthday cake she had baked (when she was struggling a bit on a pension) and having the cake taken out of her hands ans the door slammed in her face for her troubles.
DM would insist I maintained a relationship with my aunty even when aunty was being an utter cow to her, as she didn't want the family to totally disintegrate. She put up with loads. And aunty just got a bigger and bigger kick out of the cruelty and power trip angle. Told me after DM died that she didn't regret anything she'd done, one of he most important things in life is to learn how to fight dirty when you are outgunned in a fair fight.
My point is, unless your DSis has insight into her own behaviour and genuinely regrets it, wants to change and works hard to do so, you are on an absolute hiding to nothing.
So I would go NC, swiftly and simply. If any family members get involved, have a very short and truthful simple statement of explanation like "DSis treats me very badly and I don;t see signs of that changing, so it is better for me not to be involved in that situation". And stick to it.
Don't get drawn into any cat and mouse type games of reconciliation/repentance. If she males a sincere plea at any point, give her once chance. Make it clear that is one chance and what you expect to happen in order for it to work.
I NC'd with aunty about 18 months after DM died. She had decided to try to play the same games with me as she did with my mum. I mention this as, if she already has children and you are about to try, sibling rivalry can really burst into flames once another generation is involved, it can cascade down to nastiness about your children. IVF frankly is emotional and physical strain enough without that kind of rubbish.
Nc'ing was a process of gradually extracting myself from her life and then once I was strong enough after grief counselling etc, making a clear and definitive break. I heard from her once when she notified me of a bereavement in the wider family about a year later. She said some very vindictive things on the phone which were designed to hurt me and create a fight, hopefully one that would spill over into a confrontation at the funeral. I didn't rise to it, expressed my regrets politely and calmly on the phone sent flowers to the funeral, explained to others that I could trust myself to behave with dignity at that ceremony, wasn't sure about aunty, so thought discretion was the better part of valour. Then changed phone number and that's it.