Zaugol'naya. Invaders. Book 1. The Secret of the Mouse King

Theodore, whom everyone simply calls Teddy, has turned thirteen, and everything around her has changed. Her eternally busy father, who once expressed care and love through gifts and fulfilled all her whims, sending her to any clubs and sections, suddenly... became distant and found a new wife, bringing not only a stepmother into their home with Teddy but also her son. Fortunately, the stepmother turns out to be quite different than Teddy imagined, and her stepbrother Dimka becomes not only the life of the party at school but also her best friend. But this is just the beginning, as their father sends them both to their grandmother's village for the whole summer. Jumping off the electric train at a station that was not on the schedule, the teenagers have no idea that they will end up in a parallel world where mice and birds grow as tall as humans, and people seem to have landed in a mysterious valley straight from the 16th century...