When we first see Yoko (Nanami Kasamatsu), the stone-faced heroine of “Me & My Brother’s Mistress,” she’s looking right into the camera lens. Then she raises a camera of her own and starts taking photos, capturing her older brother, Kenji (Satoshi Iwago), emerging from a love hotel with someone who definitely isn’t the woman he’s about to marry.

Yoko might make a good paparazzo someday, but for now she’s just a high school student, and her promiscuous sibling is all she’s got. Orphaned nine years earlier, the pair have been living together happily enough, and Kenji talks eagerly about making his fiancee, Kaho (Hachi Nekome), part of the family.

So why is he carrying on with another woman? Encouraged by her bubbly school pal Xiaomei (the single-named Taiwanese actress Yobi), Yoko trails her brother’s mistress to a cafe and confronts her over a plate of spaghetti. But the expected fireworks never happen: The woman, Misa (Yui Murata), is both more sincere and vulnerable than Yoko had anticipated, and they fall into an unlikely alliance.