Thanks, MR.
Yeah, apart from all the crime stuff, the novel is majorly about the social politics of Montreal and specifically Westmount.
Sophie is caught between three worlds: the world of her family, a proud Québecois family that has shades of seperatism and doesn't approve of her going to an English private school (she's the only one in her family who can go to English high school, because of Bill 104 which states that if you are a Canadian citizen living in Québec, you can only go to English high school if one of your parents did three years of English education in Canada), the world of her generation, in which the more 'Americanized' you are, the better, and the world of Westmount, where wealth, family history and a big house make up for any sin.
ETA: Meanwhile, Miranda/Meredith comes from a rich Jewish family (which make up most of the Westmount upper class) and is the only girl there to have skipped her own Bat Mitzvah party. In Westmount, parents spend millions on those parties not to make their kids happy but to show off their wealth and status. Miranda/Meredith has all the background and money she could ever need to have a permanent place in the popular crowd and doesn't want it. Sophie can never have the perceived class or worth to fit in, but she tries desperately.