Big age gaps so we weren't friendly as kids but have become so as adults.
They are my go to people when the chips are down and we've had a lot of shit over the last few years as most of our relatives have died during this time period.
My sister and I holiday together and she floats around on the periphery of my social circle as I do to hers (ie. she will occasionally see one of my friends without me).
Brother lives a long way away and won't travel with us as he likes to be spontaneous and we are planners.
We what's app several times a day and I speak to them individually a couple of times a week. Interestingly they don't speak to each other that often.
SIL is joining our gang but her own family dynamic is different and she finds ours a bit difficult at times (we are quite direct with each other). I'm not sure she and I would be friends if we met in other circumstances (and we might have as we have lots of mutual friends... the world is quite a small place).