Or maybe it's just that when I was in high school, I knew who of my classmates had brothers and sisters, and who was living with one parent, and who was living with two. I even knew things about my classmates' grandparents, and that included people with whom I was not particularly friendly. A cultural difference between me and JKR? Or a sign that the novels are from Harry's perspective and that he's not particularly perceptive?
The fact that the readers don't know something doesn't mean that Harry doesn't know it. I'm sure he's known for years whether Dean, Lavender, and Seamus have brothers and sisters, for instance, it's just that there isn't any reason why a conversation where this information came up would be important enough to be relayed to the reader, unless their siblings were going to serve some sort of plot function.
We know about the Blacks and the Weasleys because they're important to the story; we don't know about the Lupins and the Grangers because they aren't, and the books are long enough as it is.
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The fact that the readers don't know something doesn't mean that Harry doesn't know it. I'm sure he's known for years whether Dean, Lavender, and Seamus have brothers and sisters, for instance, it's just that there isn't any reason why a conversation where this information came up would be important enough to be relayed to the reader, unless their siblings were going to serve some sort of plot function.
We know about the Blacks and the Weasleys because they're important to the story; we don't know about the Lupins and the Grangers because they aren't, and the books are long enough as it is.