This story is pure wish-fulfillment. For parents. For lazy parents. I'm going to be ranting here, and probably getting wildly dramatic, so check out if you're not interested in the Opinions of One Librarian.
So, while quite a few kids will probably be attracted by the bright colors of the cover, and a few more will enjoy the wacky antics of Sunny, the story as a whole does not stand together. It's way beyond unrealistic - Masha is given painkillers at the hospital and has her head shaved without parental permission, just to name one thing. I'm not even going to mention the "sick kids at the hospital making me realize how good my life really is" hackneyed device. The plot is scattered all over the place and there's no effective resolution, since Sunny ends the story by doing...exactly the same thing she started it with, experimenting on her older sister.
This is one of the things that annoys me, and this is a totally personal gripe; when siblings torture, tease, and otherwise abuse other siblings there should be consequences. Sure, Sunny is a little girl, but at age six she should have some basic boundaries and understanding of privacy. If not, Masha should have a lock on her door! Sunny is supposed to be this amazing genius, but it's extremely clear to me that she's manipulative and she knows what she's doing. The flowers glued Masha's hair isn't the first time she's run an experiment on her older sister, and judging from the final incident, it won't be the last. But their mother just tells Masha to calm down, and Sunny gets out of any punishment pretty much by looking cute and claiming that she's "just trying to help". In my book, that does not give you a get-out-of-jail free card, that gets you a stern lecture on not "helping" without permission, respecting other people's privacy, and possibly a meeting with the school counselor to discuss the concept of "professional ethics" since she's such a super-genius.
Which is why I call this story wish-fulfillment for lazy parents. A good parent would have disciplined Sunny and made sure she understood that she can't experiment on or "help" people without their permission the first time she did this. A good parent would have realized that Masha was suffering from some major self-confidence issues, not to mention the trauma of a divorce and a major move and at least made an effort to get her some help. A good parent would have, at the very least, said "hey, I can't deal with all this on my own" and gotten someone to help - after all, that's why they moved back to the mother's hometown, right? A good parent would, at the very least, have gotten Masha a lock for her door!
But this is wish-fulfillment for lazy parents, so no matter what the cute little brat does, it's ok, it's just an accident. The kids will work out all their feelings over the divorce, have a little sisterly bonding and some wacky hijinks, and in the end they're sisters and they love each other no matter what, they both find new friends at school, and everything is hunky-dory. No parenting needed! You can go off and do whatever it is you prefer to be doing rather than actually parenting your children!
In real life, Sunny is going to pull one of her nasty experiments on someone other than Masha and their family is going to get sued for invasion of privacy - or at the least, Sunny will be suspended or expelled. In real life, Masha is going to request that her custody gets switched to her father and never speak to Sunny again. In real life, Masha will get angrier and angrier at Sunny, at her mother, and life in general and probably end up either in therapy, with an ulcer, or on drugs or all three. Because life is not a Disney comedy and books shouldn't be either.
Verdict: Wish-fulfillment is fine, suspension of disbelief is great, and everybody loves wacky hijinks and pranks. But there's a fine line between socially awkward and immature children doing silly things and extremely intelligent children learning how to manipulate the people around them and consistently doing so, over and over again. A lot of kids probably would find this funny, and like I said it's an attractive cover. There could definitely be a feeling of "well, at least my sister isn't that bad". But, frankly, the book isn't good enough for me to get over my feelings of outrage. You can say I lack a sense of humor about pranking (you're probably right, although I firmly hold to my belief that if everyone's not laughing it's not funny) but there are plenty of other humorous books, books about sibling rivalry, even books about a super-intelligent younger sister, that are better choices than this book.
ISBN: 9781599909776; Published October 2013 by Bloomsbury; ARC provided by publisher at ALA Annual 2013