A book where a twin believes she is her dead twin?

[Rosemary]

I'm looking for the title of a book I read within the last 3-4 years. I was a fiction book, modern setting, about a young woman who meets a guy, starts a relationship and then her twin sister who is less of a 'good girl' shows up. The guy then has an affair with the 'bad twin' whilst still seeing the good twin, and the whole thing reels out of control. It then seems like the bad twin has killed the good twin (maybe?) and is running away but eventually it transpires that the 'bad twin' is in fact dead and she's having a serious breakdown impersonating her twin. Any idea what this book is?!

[Jessica (The Psychotic Nerd)]

Maybe Vanishing Girls?

[Jo]

Or Beside Myself by Ann Morgan?

[Juels]

We'll Never Be Apart by Emiko Jean?

[Lobstergirl, au gratin]

YA?
NA?
Adult?

[Rosemary]

Hmm, these all sound like great books but it's none of these. I think the book in question is an Adult Fiction book - I picked it up in the Fiction section at the Library. I remember that the 'good' twin worked in some kind of community centre. The twins are in their 20's I think, certainly 'young adults' in the early years of careers, not settled down yet etc.

[Rainbowheart]

Was it a UK book?