This hasn't really been my experience. My elderly father had minimal contact with his two surviving siblings even after my mum died. There were no big fallings out or anything - they liked each other. They just weren't significant parts of each other's lives. All of his siblings are dead now, which is sad, but honestly, it makes no real difference to him on a daily basis.
Of course, there is a lot of variation. My mum remained close to her brother until the very end, but I don't think it's a given.
I do support an elderly aunt who has no children of her own, but realistically, what I can do for her is very limited because I also have to care for my dad.
Ageing is hard no matter what your situation is. Having children with whom you have a good relationship is probably the strongest protective factor, but even then, there are no guarantees. Siblings may grow apart, be scattered around the globe, die young, suffer from dementia and not recognise you etc. I certainly don't see my sister as any sort of insurance policy to keep me company in my old age, and I very much doubt that she sees me like that either. Having a good, local network of friends probably makes a much bigger difference from what I've observed.