What happens in Wilderness by Roddy Doyle?

Johnny and Tom are two ordinary dublin boys. They live in a house with their mother Sandra, father Frank and halfsister grainne. Grainne is a moody teenage whose mother walked out on her at a very young age. Grainne has always dreamed of meeting her mother and finally the day arrives. Sandra decides that she doesn't want Johnny and Tom around when grainne's mother is visiting so they go on a dream holiday to Lapland, leaving behind Frank and Grainne. When grainnes mother arrives they don't get off to a great start. Elsewhere Johnny and Tom are exploring around lapland meeting wonderful dogs with amazing eyes. Everything is great untill their mother goings missing in a terrible sledding accident it is up to johnny and tom to go and find her.