I've been NC with a sibling for many years. I don't regret it per se because the peace of mind I gained by not having to deal with a self centred dishonest spiteful and manipulative bully any longer was a huge relief. I also didn't want my children to be exposed to this person's company.
The situation does, however, cause me a lot of heartache still ..... not because I miss them but because I wish I had a decent normal sibling I could be friends with. My immediate family is very small - my lovely dad died years ago, am not close to my mother though we maintain a relationship of sorts (quite distant and superficial, she seems not to have any real interest in me) and extended family are scattered far and wide and I see them very irregularly. In short, I feel quite alone and confess I am extremely envious of people I know - and at times it feels like everyone I know (in real life) - who have loving close families. Feeling I had no real choice but to cut out someone who was affecting my mental health was a necessary thing but nonetheless horribly frustrating and sad because my family is so small.
I've never been tempted to reconcile - despite my mother muttering along those lines every so often ...... I have no idea if my sibling's 'changed' in the meantime and would be too nervous of finding out. I hear bits of stuff every so often which makes me think they remain the personality I knew 'before', and nothing which makes me think 'maybe I've been too harsh'. In fact, when I initially broke contact, a lot of stuff I had no idea about came to light via cousins, aunts, uncles and so on who all held a similar view of my sibling - but who hadn't wanted to rock the boat, upset my mum etc., which reassured me I wasn't being unreasonable.
I would love to have a sibling to love and vice versa but it's never going to happen.