Magic Tree House #36
Blizzard of the Blue Moon
They get sent on a mission from somebody called Merlin. Merlin sends them in a magic tree house back to New York City to rescue a magical unicorn. When they get stuck in a snow storm. When there is suddenly a couple of mysterious people following Jack and Annie. When they stumble upon a mantion of an old man.
The characters are Jack and his younger sister Annie and an older man called Merlin.
The book Magic Tree House #36 Blizzard of the Blue Moon is written by Mary Pope Osborne.
Places to buy
You can find the book on amazon or at any close book retailer.