Sister Trouble by Linda Chapman, Kim Barnes

In the 15th installment of the Star Friends series, Lexi and her younger sister, Lily, are determined to outperform each other to win a baking competition. But when the sibling rivalry gets out of hand, the Star Friends suspect dark magic has invaded Westport....

Lexi and her younger sister, Lily, are constantly in competition with each other. They both are determined to win a spot on a junior baking TV show, and even when Mia has a troubling vision about their beloved town, Lexi refuses to let it distract her. But when all her practice baking goes wrong, and the sibling rivalry with her sister reaches troublesome new heights, Lexi and the other Star Friends wonder if dark magic has invaded Westport…. With black-and-white illustrations throughout.

In the Star Friends chapter book series, Mia and her friends Lexi, Sita, Violet, and new friend Maddie all discover that they are Star Friends -- they can communicate with the animals that have traveled from the Star World. The animals show the girls how to use their unique magic abilities to do good. A whole world of magical adventures awaits!