Thing is you have no idea whether you'll manage a second-nor if they'll get on, or if they'll be there when needed.
Friends of my parents died in the last year. They had 2 dc, one of whom died 3 years ago in his 40s (unmarried) and the other one is single too. She's now very alone, and, I think, feels so, literally no family at all. But she had a sibling growing up.
For me, I'm the middle one of three. It has varied.
Up to age 10yo I was close to my dbro.
Then I couldn't stand him and was close to dsis.
There was a period I didn't really see either, and then I got really close to dsis.
Unfortunately something has come up recently and I've realised that dsis is really (I think) jealous of me and I'm not quite sure how to handle it. I don't think she should be, but I am aware (I don't know if she knows I'm aware) of a massive strop she had which upset dm, which I think jealousy is at the back of it. I'm saddened by it.
Dsis and dbro have always I would describe as neither getting on or otherwise. They just didn't share any interests, were quite a difference in age, and I have no recollection of them doing anything together just them. Genuinely cannot think of a single activity or interest they ever did together even as a one off, let alone regularly. Nor even really joining in things as a family, it tended to be one or the other.
Which made me not sure what to say when dbro's wife said to me that if she had children she hoped they'd be as close as dbro or dsis were because they had obviously been so close as children. Dbro had a slight
look on his face, so I suspect dsis has rewritten history in talking, which leaves me a little
as to what to say. It's come up a couple of times (never when dsis is present, but I don't often see both at once due to distances anyway, they see each other much more than I see either) and it leaves me feeling awkward because I don't like saying "oh yes, they did" because it really wasn't true, nor do I want to say it wasn't true because I feel it's a bit mean on dsis. so I change the subject.
What I would say my observations from friends who were only children is that the parents seemed to have more difficulty moving them from child status to adult. They either had them as confidant at 8yo hearing all the ins and outs of relationships and expecting them to help make adult decisions. Or they were still treating them like a child at 21yo, and expecting them to be in by 6pm for dinner.
I would say have another child because you want them. Not because you feel you should for ds.