Rule of threes / Marcy Campbell.

Enhanced descriptions from Syndetics:

Part family drama, part contemporary thriller, Rule of Threes is a middle grade story about what happens when "perfect" is out of your control.

An interior design enthusiast, twelve-year-old Maggie Owens is accustomed to living her life according to her own precise plans. But when she learns about Tony, a mysterious half-brother her own age who needs a place to stay, any semblance of a plan is shattered. Tony's mom struggles with an addiction to opioids, and now she's called upon Maggie's dad--who is also Tony's dad--to take him in. On top of everything, Maggie must also come to terms with the Alzheimer's afflicting her beloved grandmother.

While Maggie can strive for--and even succeed in--a picture perfect design, when it comes to family, there is no such thing as perfection. To work through the sudden struggles rocking her world, Maggie must learn the importance of having an open heart.

* A heart-wrenching, authentic, and darkly funny coming-of-age story
* Addresses hard-hitting topics--such as divorce and substance abuse--with care and honesty
* From Marcy Campbell, author of Adrian Simcox Does NOT Have a Horse

Merci Suárez Changes Gears meets The Thing About Jellyfish in this compulsively readable novel.

* Perfect for ages 10 and up
* Appeals to those interested in stories about friendship and non-traditional families
* Great for readers who love a bold and self-aware protagonist