Damn, you so promised we wouldn't get deep!
Can we not both be right? We are talking about a little girl, and for all her coolness, she's a child, who is revealing her life as it is. Sure, it's filtered through her point-of-view (I must say that I didn't notice the POV shifts as such, which means I was deeply immersed in the reading; I tend to notice craft while reading when the author is failing miserably), but it's a pretty bald recitation of facts.
One thing I did notice is that this story does not, like many novels of this ilk, come off as a "how I became who I am" story. We are not looking back; we seem largely in the present throughout. This isn't a case of a someone lamenting over the scent of cookies, and I think that makes this a particularly fresh look at, well, life.
But as to the question of sentimentality versus sentimentality, our narrator loves her family, deeply. Even her father, maybe even his slut. In her recounting of events, she's oddly protective of her mother. You can be, I think, factual (such a fluid word, makes one love English) and sentimental. I think it's one of the things that makes domestic drama so powerful. You will say anything and everything behind closed doors, but there's a public face that must be maintained.
I find that some of the smaller characters remain with me. Bonnie's boyfriend. Could have been so much more perverted than he was. The Slut, who sometimes seemed to be flailing in a pool she didn't mean to enter. Ma Mere, who, ultimately, was just an old lady and couldn't explain that to the kids who were scornful of her.
Circling back to craft, the story's style is skillfully maintained. Matt discussed the fragmentary structure of the style, and that's a large part of it. Murphy's voice is spot on as well. The first person narrator, who carefully speaks of "we" and talks about "our" various things. It is the voice of a child because it is, to me, the voice of a child parroting the words of her parents. We do this, we do that. We mustn't bother our brother. We should ask our mother. Did you notice the style right away or did it just flow into you naturally?