An Escape in Time by Sally Nicholls (In Time #3)
In the hallway of Aunt Joanna’s house, there was a magic mirror. Huge, gold-framed and mysterious, there it hung, looking innocent. Ruby Pilgrim glared at it. “Just look at that!” she said indignantly. “Sitting there like a lump of glass! Like it’s just – a reflecting thing or something!” “It is a reflecting thing,” said her brother, Alex. “It’s just … sometimes it’s a time-travelling thing as well.” Because sometimes, the mirror showed another reflection, of another Applecott House in another time. Last summer, they’d gone back to 1912 and helped save a priceless golden cup. Ruby still hadn’t sorted out how she felt about that. She’d hated it and she’d loved it, all at the same time. It had been wonderful … and really, really frightening. For quite a lot of it, she’d been certain they were going to be stuck in 1912 forever, and would probably have to go and live in a workhouse or something and…