Your lovely post about sisters in picture books made me think immediately of a couple of new additions to our bookshelves, which aren't technically about sisters, but have very much a sisterly feel.
I'm talking here about Bink & Gollie and its sequel, Bink & Gollie: Two For One, by Kate DiCamillo and Alison McGhee, illustrated by Tony Fucile. I mention the illustrator right off the bat because these books are so illustrated as to feel almost like graphic novels, though their structure is more early-reader: each book contains three stand-alone stories, loosely linked.
Bink and Gollie are best friends: Bink is the small, wild-haired blonde; Gollie the tall, thin, and hyper-articulate brunette. You can imagine how easily my own tall, brown-haired Eleanor and smaller, blonder Isabel see themselves in this pairing. There is no mention of either girl's family; though they are clearly kids, they seem to live in small individual houses close to each other. Bink's house is classic one-room-house-shaped, while Gollie's is a far more modern structure, spare of furniture, perched up in the top of a tree. The setting feels a little like the houses of the various animals in Winnie The Pooh, an impression that grew on me as I noticed each girl pretty much makes and eats only one thing (peanut butter sandwiches for Bink, pancakes for Gollie).
The stories in the first book involve individual desires and ultimate compromise: Gollie detests Bink's new brightly-colored socks, and refuses to make her pancakes until she takes them off; Gollie is on an imaginary adventure climbing the Andes Mountains, but Bink keeps disturbing her when she wants to be alone; Bink gets a new fish and spends so much time caring for him that Gollie gets jealous. Very sister-like, in the ins and outs of negotiating play together.
What this summary misses is the pleasant loopiness of the dialogue, powered by Gollie's slightly formal tone and vocabulary and Bink's straightforward responses:
"Bink," said Gollie, "the brightness of those socks pains me. I beg you not to purchase them."
"I can't wait to put them on," said Bink.
They refer to themselves as "marvelous companions."
In the second volume, Bink and Gollie visit a state fair, where Bink tries to throw balls to win a giant donut, and ends up hitting the guy running the stand; Gollie attempts to recite a poem at a talent show and is stymied by stage fright; and both girls go to see a fortune teller. A similar sense of sisterly support permeates these stories as well.
Love to your big sisters from my little ones,