Puzzled by the first half of Jansson's The Summer Book

In one day's reading I'm halfway through Tove Jansson's The Summer Book and I know it's short but maybe I should be reading more slowly and carefully because I think I'm missing the point. The novel consists or short chapters, each something like an essay or observations, about a young girl of very indeterminate age - sometimes she acts like she's 5 and sometimes like 15 - and her feisty and independent grandmother, living on an island in the Gulf of Finland (time uncertain, maybe mid 20th century?). The girl's father is with them but plays almost no role in this; we learn almost in passing that her mother recently died. There's so much that's unclear, starting w/ why are they on the island, is it a summer home or do they live there year-round? At first it seemed like it was a tiny island with maybe only a few families, or maybe only 1, but gradually we learn that there's a village on the island and a number of year-round settlers. A few neighbors make appearances - but the heart of this novel, if that's what it is, remains opaque and there seems to be no arc to the story: the chapters don't necessarily proceed in succession, the girl doesn't seem to be growing or maturing or overcoming any crisis or sense of loss. Does it take place over the course of one summer? Many? What is the grandmother so harsh? The the young girl have any relations w/ anyone but the grandmother over the course of this summer? In some ways The Summer Book reminds me of Strindberg's Natives of Hemso, an obvious point of departure from any Swedish writer, but his novel was so much more finely developed and attuned to the entire culture and the changing ways of the lives of those on an island in the Stockholm archipelago. The Summer Book is to me just confusing, so far - a series of starts, with no direction home.