ABANDONED. Children's/Juvenile book about three siblings with one of them narrating

[Elsa]

The cover of the book had three children standing beneath a tree: a tall, lanky blonde boy with a gray scarf covering his nose and mouth, a girl with long black hair and a pink headband, and a shorter boy with brown hair.
In the book the children are orphans. The blonde boy is the oldest at ~ 16 years old and rarely speaks, his name is Oslo or something along those lines. The girl is named Lucy and is described as having "shiny" long hair that "took lots of shampoo to lather in the shower" and having a crush on her English teacher. The younger boy is around 13 or so and I don't remember much about him.
I think they were left in the care of like their uncle and at some point in the book there is a rumor described that the blonde boy "strangled their mother with that scarf in a fit of rage" when he has an outburst and beats a heavily tatooed man with his camera for taking a picture of him.
The narrator is one of the three children and complains that "I'm doing all the work" in telling the story but is unable to divulge which of the three children he/she is. They say "you're welcome to guess, of course."

[Elsa]

I read the book around 2008-2013 and I remember it gave me a kind Series of Unfortunate Events-ish vibe. I'm guessing with was published around 2004-2010??

[Rainbowheart]

A Series of Unfortunate Events vibe kinda reminds me of Philip Ardagh, but I don't think it's The Fall of Fergal or Awful End.

Was the book directed more at kids or teens?

[Mai]

Hi! The description of the cover made me think of this book I found for another search:
https://www.goodreads.com/topic/show/...

The book was The Kneebone Boy by Ellen Potter.

[Kris]

Elsa, Mai's suggestion of The Kneebone Boy by Ellen Potter matches your description. Here's the Google Books preview:

https://books.google.com/books?id=5in...

From the book: "There were three of them. Otto was the oldest, and the oddest. Then there was Lucia, who wished something interesting would happen. Last of all was Max, who always thought he knew better." | "People whispered that ____ had strangled his mum with that very scarf in a fit of rage."

[Kris]

No response, moving to Abandoned folder.

Elsa (OP) was last active on the site in January 2020.