Bittersweet a novel

This is quite an odd book and it is difficult to interpret both the author's intentions and the motives of Mabel, the first person narrator. The whole book is from the point of view of the ugly duckling Mabel who becomes friends with the glamorous Ev and her family. As I grew more and more frustrated with the utterly self-obsessed Mabel I began to wonder if the author was portraying her as an unreliable narrator and if we are supposed to dislike her? She certainly turns out just as unlikable as any of the family she exposes. Everything she discovers is pointed out in VERY BIG LETTERS, particularly the incest, like the readers are unable to draw their own conclusions. Birch is flagged up as a bad guy from half-way through the novel by his cruelty to the dog, from there he is only a tiptoe away from becoming a Nazi Art looting father of incestuous children.
Anyway, the first half increasingly reads like a YA novel about the relationships between the young people, and I got increasingly impatient for the mystery element, as all Mabel seemed to be doing was mooning over Ev and Galway (was it me or did he just seemed to be sort of tagged randomly with Mabel? All of a sudden he showed up and fell in love with her even though they had nothing in common other than her creepy obsession with his family?). The second half veers into overblown Gothic with the discovery of the incest, art looting and the murder, up until the horrible end where Mabel marries rescues Lu, marries Galway and the bad guy drowns. It just seems a bit, well, preposterous that everything get tied up so neatly and Mable ‘wins' becoming part of the Winslow family.
I think I judged this harshly because with all the Paradise Lost references and some of the techniques she used such as deliberately obscuring the time setting, I was expecting this to be a bit more literary. As it was, I found it quite frustrating to read.