Missing Okalee / Laura Ojeda Melchor.

Enhanced descriptions from Syndetics:

When compared to her nearly perfect little sister, Phoebe Paz Petersen feels she doesn't measure up in her parents' eyes. Okalee is smart and beloved for her sunny disposition, which makes it hard for Phoebe to stand out in their small town in Montana. But if she can get picked for the coveted solo in the school choir, she'll stop being a middle-school nobody and finally get her chance to shine.

Despite her sister's annoying perfection, Phoebe actually loves spending time with Okalee. They have one very special, secret tradition: River Day--when they hold hands and make their way across the cold, rushing Grayling River, to celebrate the first hint of spring. This year's River Day crossing, however, goes horribly wrong, and Phoebe's world is suddenly turned upside down.

Heartbroken and facing life without Okalee, Phoebe is more determined than ever to sing the solo in the school concert as a way of speaking to her sister one last time. But Phoebe's so traumatized by what happened, she's lost her beautiful singing voice.

Kat Waters wants the choir solo for herself and is spreading a terrible rumor about what really happened to Okalee on River Day. If Phoebe tells the truth, she believes her family will never forgive her and she may never get to sing her goodbye to Okalee. Even worse, somebody is leaving Phoebe anonymous notes telling her they saw what really happened at the river.