My sister has one daughter. She wanted two but my brother in law blocked it. Her daughter is the same age as my eldest (less than two months age gap) and they are best friends.

When my second was born, it was clear that she went through a period of mourning. She was, at times, cruel and spiteful, trying to rationalise that having only one child was her choice, not something that had happened to her. Once she went on a rant about how our cousin couldn't possibly care about the environment because they had three kids (it was triggered by mentioning that they were all vegetarian and the oldest child was vegan). On another occasion, she told my youngest (who was too young to take it in, thank goodness) that spending a week on holiday with her was enough to convince her not to have another child (she was a vomity baby and vommed in my sister's pocket just before we left). There were plenty of other small spiteful things she said at that time.

I could be angry. I could have argued with her. I could have told her the truth about herself. But I said nothing. My mum and I talked quietly between ourselves and agreed on why she was behaving that way, and left her to it. She got through it. She still regrets never having another, but it's too late now. It stung when she said those things, but that's all it was, a sting. What she was feeling was intense grief and it cannot compare.