We don't see much of the other Kinkaids. All we know is that Kevin is a brat and the four siblings aren't all that close:
Usually, the four Kinkaids neither waited for each other nor walked together, except for Kevin, who was somebody's charge each week. (1.13)
The Kinkaid family isn't very present in the book, even though they're supposedly part of what Claudia is running away from. Hmmm. Maybe it's the disinterest and blandness of their family life that makes her want to be different, to stand out.
It's also probably why they adopt Mrs. Frankweiler at the end. She's interesting and old and needs a family—and they're little and bored and need an older mentor who can understand their thirst for adventure. The three of them make a pretty good match, don't you think?
In contrast to the rest of the Kinkaids, Claudia and Jamie's close sibling relationship is made to look even stronger. And when the kids come across Mrs. Frankweiler, it's obvious that their relationship with her is as important and as strong as some of their familial ties. All together, they form a bond based on art, adventure, and a whole lot of secrets.
And that's the kind of stuff that has some serious sticking power, whether you like it or not.