Not as uncommon as you might think op, a lot of the time people simply don't advertise the fact.
I am nc with my sister and have been for several years because she's a narcissistic, cf, abusive bloody nightmare! But very few people know the whole story, most just know we're "not close"
I'm barely in contact with bro, no major issues just not much in common and don't live near each other.
My ex hasn't spoken with his eldest brother for many years even I don't know the full story
My dad and his siblings are CONSTANTLY falling out over various things, he barely speaks to any of them these days as there was a major fallout over an inheritance issue a few years back. But they've always been like this to the point that for my wedding I put each sibling on a different table!
My mum and hers mostly get on but they have their moments too and 1 in particular the other 5 don't tell em anything sensitive cos they either blurt it out to wrong person or use it against them!
My ex in-laws each have one sibling they and other siblings don't speak to both from large families.
Out of my friends there are several who just play it "not close" but I suspect estranged and a few I know are completely estranged/nc.
As a contrast I also know a family where the 3 sisters are frankly imo too close and I wonder why! They make a huge deal that they're each other's "best friends" but they're not at all close to anyone outside of the family and I know for a fact it's caused issues in their relationships inc one marriage breakdown as he just had enough of everything being told to the sisters, them knowing EVERY intimate detail about his marriage and his family not even being on a list of prioritised people.
Extremes in both directions
I don't know anyone in RL who doesn't get along with their sibling
I would suspect either you don't know many people or the people you do know who don't get along with siblings simply haven't told you. I have quite a few friends/acquaintances who have no idea that I'm nc with my sister, it's none of their business and I tend not to tell people because I've already experienced supposed friends who do get along with their siblings being horrified and judgmental about the situation when they didn't even know the whole story.
Unfortunately my sister and I look very alike albeit different colouring but enough people who have met us both can see that we're sisters and we live in a very insular location, so I've frequently had people assuming we are close and passing comment about the other. To the point where it's been people in certain positions/professions where this has been the case I have uncomfortably felt the need to tell them so that they don't tell her stuff I don't want her knowing! It's a right PITA!
@DorotheaHomeAlone you contradicted yourself by saying you know of a couple of cases plus as I said many people simply don't advertise the fact
I remember years ago I was friends with someone and didn't even know they had a sibling until they lost a parent - I'd even met the parents that's how good friends we were - and they then explained the situation and why it was making them so anxious about the funeral. They had good reason to be the sibling behaved appallingly at this time inc at the funeral.
@Witchend ALL the things you advise - my parents did the exact opposite!
I tried to get along with my sister but over the years she's got worse and worse and it reached a point neither I nor Dd were safe near her to be honest it was that bad!