http://www.amazon.co.uk/Sophie-Kinsella/e/B001H6NQJITo the right, if you scroll down a bit, there is a bit which has a selection of authors which people who have bought Sophie Kinsella books have also bought. Lauren Weisberger wrote
The Devil Wears Prada and the books which are a part of that series, which may be an option. I've only read a couple of Cecelia Ahern books off that list, and enjoyed
Where Rainbows End. Those authors are fairly similar in style to Sophie Kinsella, I think. (Decent is subjective, but they are popular authors within that audience who write in a similar style, so they must be doing something right
)
I wouldn't have said
Atonement is really in the same league as Sophie Kinsella, but might be an idea if you wanted to widen the range of books your sister reads, although it might be a bit of a difficult read to begin with! What about
The Time Traveler's Wife? It's not quite the same style and may be a little bit difficult to get her head ound, but lots of people rave about it, including friends of mine who have really enjoyed Sophie Kinsella. Or Jodi Picoult? Again, not really the same, but do tend to have a romantic plot within them as well as the main story. My favourites were
Nineteen Minutes,
Plain Truth and
My Sister's Keeper.
If you have a look on Amazon at Sophie Kinsella books, there are always recommendations at the bottom based on what other people have bought though, why don't you have a scroll through those and see which ones have got the best star reviews and ratings, and maybe pick something she wouldn't necessarily have picked up, if you get stuck?
Good luck!