Inspired by this article in the Times:
https://www.thetimes.com/life-style/parenting/article/why-cousins-are-dying-out-it-matters-more-than-you-think-d6vr0rgtx
It's behind a paywall sorry but basically it says due to declining birthrates children born now are much less likely to have lots of cousins. I think down from average 7 to 5 but falling quite a bit. The article argues that this means children have less peers their age in family gatherings to play with and smaller networks.
I find it very interesting as I have 13 cousins but I'm not very close to any of them as an adult. My DC have 4 for now but probably 7 or 8 once everyone is done. I'm quite glad they have quite a few cousins but perhaps we are becoming more unusual as all our siblings have kids or intend to.