Basically when Anna and Elsa were kids, when the tolls were removing Anna’s memory of magic, Elsa interferes and they end up cursed. If Anna is anywhere near Elsa, she’ll turn to ice, and Elsa’s ice powers went dormant. So they have to be kept apart, with no memories of each other. So all memory of Anna’s existence has to be erased from the kingdom and she’s sent to grow up with another family in a small village. Elsa only rediscovers her powers and starts remembering Anna after their parents die. So the book proceeds similar to the movie, with Elsa’s powers being discovered and the kingdom being thrown into winter, but both Anna and Elsa are trying to find each other. Elsa, because she just found out she had a sister and Anna just feeling like she had to do something to help. Then it ends with the same happy ending as the movie.