ds is an only child. My Mother was also an only child. It was funny, sometimes, seeing her try to figure out what the hell was going on between my and my sister.
She really didn't get the complications of a sibling relationship but worked out that if she intervened and appeared to be unfair to one side we would both gang up against her - it was only when I was an adult I discovered it was a deliberate tactic to stop the endless bickering!
I love my sister dearly as an adult and that relationship is very precious to me. I hope ds might have something similar with his cousin, only two years younger and also an only.
As for being lonely as a child, yes, I can see that might happen, but we live in a street where kids play out, ds goes to after school club, and my niece is often round, so he gets plenty of chances to play. When I worked largely from home I had children round to play at least three times a week.
When we go on hols, we usually take my niece. Most recent hol she couldn't come, and I did notice the difference, as ds didn't make any friends with the other kids in our hotel - they all had their own siblings to play with. In future I think I'll look for holidays with activities that ds can join in.
I also have a half-sister who we used to be very close to, but an awful family row means we don't talk any more. Very sad but shows that just because you are related doesn't mean you will get on.
Dh has also spent most of his adult life not having much to do with his sister - no real reason, they are just very different people with very different interests. What's lovely is that as his mother got increasingly infirm, needing more and more care, dh and his sister have worked together really well and become much closer. It's a sad situation but lovely that it's bought them closer when I know it drives so many siblings apart.