I loved Caroline B Cooney when I was in my teens, maybe a bit older than 12 but kids grow up fast these days. She may be a bit 90s though...! "The girl who invented Romance" I seem to remember enjoying...

Summer of My German Soldier is a subtle romance for that age.

The Time Traveler's Wife by Neffinger was one I enjoyed very much.

I loved "The Blue Castle" by L.M Montgomery. Also, how about "Rose Daughter" by Robin Mckinnley? Anything by Jane Austen may also be a good choice.

Shiver by Maggie Stiefvater is all I can think of right now, but I remember liking Summer of my German Soldier, and I agree that she might enjoy Jane Austen.

Read Ivanhoe it's a romance.

Any book by Janette Oke is very mature. And all of her books are great.

The Scarlet Pimpernel
Beauty by Robin McKinley

The Little White Horse by Elizabeth Goudge. Romantic in a way that's appropriate for her age, plus has mystery, family feuds and magical animals. I still read it every year as an adult!