For a lot of people, writes Angela Chen in The Atlantic, “[s]ibling bonds are the longest relationships of our lives. We know siblings before we meet our partners (and before we have our own kids), and we’ll know them after our parents die.” 

Some research even suggests that siblings have a higher impact than parents on whether teens do drugs and alcohol. Another study found that “subjects who had conflict or distance in their relationships with siblings before age 20 were more likely to be depressed at age 50.”  

What G.K. Chesterton once wrote about neighbors describes siblings even better, 

We make our friends; we make our enemies; but God makes our next-door neighbor. Hence, he comes to us clad in all the careless terrors of nature; he is as strange as the stars, as reckless and indifferent as the rain. 

The fewer children that Western couples have, the fewer siblings there will be in the world. And that will be a poorer world indeed.