Every set up is different, you can't know until you're on the inside of each family I don't think.
My mum has a brother but when their mum started suffering with dementia it was my mum who did everything while he stayed well away. Me, my brother, my dad and my dad's siblings were her support. She doesn't speak to her brother now because she can't forgive how he left everything to her.
My dad has a brother and sister and they are all pretty close.
Me and my brother were very close growing up and although we're there for eachother now, it's more distant unfortunately.
My husband has 2 siblings but they're much older and he feels very seperate to them.
We have a son who is 4.5 and very unlikely to have another. He was "third time lucky" after 2 miscarriages. During lockdown and the aftermath it just wasn't feasible for us to consider trying - family deaths, redundancies, severe sleep deprivation. Ds is now awaiting an autism assessment, which we've been told by nursery, HV and gp will be "a formality". Our days can start at 6.30 and not end until midnight. Or they can begin at 2.30am. But, he's awesome. Loves me and his dad and his grandparents, who are able to help out with him but could never manage him AND a sibling. He has a couple of friends and loves seeing them, but is always ready for everything to "go back to normal" at home afterwards. We've had a holiday recently just the 3 of us and it was fantastic! Went to the zoo, birdwatching, the beach. Film nights with pizza. Ice cream on the pier in the evening when it was quieter.
I'm not sure how he'll get on as an adult, as me and DH age etc but I also know that sibling is no guarantee of anything.