My siblings and I are really close, there's 5 of us altogether. Each one of us have different personalities and different perspectives in some things but also many similarities. I'd say I'm closest to my third sister and then our other two sisters are closer but altogether we love eachother and mesh well. I was very close to my brother too until he got married to a very toxic woman, now we're OK but not as close as before.
Tbh, my siblings are the best gifts my parents could have ever given me!! I don't really have many friends per se as my siblings are my best friends, apart from my dh they're the first people I contact for everything and we're always just there for eachother, even after fall outs and arguments we just carry on as normal after a couple of days as of nothing happened because we know that it's petty silly stuff.
My dh is one of 10 siblings and they're all close too, I guess it's our culture too. We're from a middle Eastern culture where family is everything really, including wider family like cousins, aunts, uncles etc so we're all pretty tight knit.
That's not to say we never argue, but we always sort it out in the end, cos we're family and can't get away from eachother so gotta get on or life would be pretty miserable!!
Right now my eldest two can either be best friends or worst enemies, there's no in between, I hope they're all close when they grow up too...