I got this ALL the time for a few years, even though I had two (“everyone” round here has three). It really really pissed me off. Especially if I’d spent a few moments politely batting the comments aside, and then they still carried on. In the end I took to saying “well, I could have another, but then I’d probably have a stroke, or die, and leave DH with 3 to look after, and we don’t want that do we?”. That worked.
The problem is that people always assume they know what it is like from the perspective of a child with siblings. Lots of kids really never do get on with their siblings, obviously lots do, but it’s not a given. There’s a big gap between mine, and I made sure they went to nursery early as possible, lots of friends around, etc etc... but also because my experience is not that my sibling is my best friend.
Too long, but this is a triggering subject for me 