This is Sarah Mlynowski’s Whatever After book 11, Two Peas In A Pod!
This series is about a girl named Abby, her brother Jonah and her dog named Prince. In they’re basement they have a magic mirror, where a fairy was cursed inside it! On occasions, at mid-night, if you tap on the mirror 3 times, she might just send you into a fairy tale! A book for 7-11 year olds, an easy read, and so much fun. Sail on, Readers!