The theory of (Not Quite) Everything by Kara Gnodde

With the offbeat charm of The Rosie Project and generous warmth of The One Hundred Years of Lenni and Margot, a wry, moving debut novel about a pair of unforgettable siblings and a love triangle of sorts—one with math as its beating heart.
One of Cosmopolitan’s Best Books of 2023
Meet Art and Mimi Brotherton. Devoted siblings and housemates, they’re bound together by the tragic death of their parents. Mathematical genius Art relies on logic, while Mimi prefers to follow her heart.
When Mimi decides she needs more from life than dutifully tending to her brilliant brother, she asks for his help to find love. Art agrees, but on one condition: that she find her soulmate using a strict mathematical principle. Things seem promising, until Mimi meets Frank: a romantic, spontaneous stargazer who’s also a mathematician. Despite Mimi’s obvious affection for the quirky Frank, Art is wary of him from their very first encounter.
As Art’s mistrust of Frank grows, so do Mimi’s feelings, and the siblings’ relationship is brought to a breaking point. Something about Frank doesn’t quite add up, and only Art can see it . . The Theory of (Not Quite) Everything is a tender, intelligent and uplifting novel about brothers and sisters, true love in all its forms, and how the answers to life’s biggest questions follow a logic of their own.