Your sister-in-law’s comment wasn’t just a hurtful insult—it was an act of aggression. In response to your question, she went right for the jugular with the cruelest possible thing anyone could say to a parent. This makes me think that she wanted to hurt you because she felt hurt by you.
You might be wondering how you hurt her, and my hunch is that your question about why she didn’t have a second child hit on something painful for her. Of course, it’s possible that your sister-in-law is just a horribly insensitive person, but usually people lash out in this way when you touch something tender in them, and to get rid of their pain, they toss it onto others.
Although I’m sure that you didn’t intend to hurt your sister-in-law by asking why she had only one child, your question might have been a loaded one for her. A common lament I hear in therapy from people who have either one or no children is this: Why do people ask those with one child why they didn’t have another, but don’t ask those with two or more children why they didn’t have another child—or fewer children? Why do people ask those without children why they didn’t have them, but don’t ask those with children why they decided to have them? The implication is that if you have no children or one child, there has to be a good reason (infertility, not enough money), because most people would choose to have at least two. For those who wanted multiple children but didn’t have them, questions like these can be incredibly painful and even create a sense of shame.
Shame is at the core of what’s called a narcissistic injury. A narcissistic injury occurs when a person reacts to a perceived criticism (about, say, having just one child) with rage, often in the form of a vindictive response. Whereas the average person might be tempted to say something unkind if you touch a sensitive nerve, a person with narcissistic tendencies (such as being demanding or acting as though she’s better than you) will verbally eviscerate you. If your sister-in-law has been made to feel “less than” for having only one child, making you feel “less than” for having a child with some challenges allows her to send her self-hatred your way. By making you feel small, she gets to feel big again. The result is that you are left holding the pain.
So now that you’re holding this pain, what can you do to diffuse it? You say that you don’t want to create a problem, but the problem is already there. The question isn’t so much about whether to bring it up, but how. One thing to consider is what you hope to gain from a conversation with your brother. For example, do you imagine that his sympathizing with your pain and agreeing that what his wife said was awful will make you feel better—seen, heard, understood, loved? Do you want to ask him whether his wife’s comment was untrue—that they didn’t have a second child for a reason other than their fear that the child would have developmental delays? And if there is some truth in it, how can the two of you talk about this in a way that separates his or his wife’s concerns about raising a child with some additional challenges from the fact that you have a child with these needs whom you love dearly?