The Way Past Winter (paperback) by Kiran Millwood Hargrave

Mila and her sisters live with their brother Oskar in a small forest cabin in the snow.

One night, a fur-clad stranger arrives seeking shelter for himself and his men. But by the next morning, they've gone – taking Oskar with them. Fearful for his safety, Mila and her sisters set out to bring Oskar back – even it means going north, crossing frozen wild-lands to find a way past an eternal winter.