This request is from Kim, who remembers more of the mystery elements than the romance elements of this book:
The book I'm trying to find was a romance/mystery from the mid to late 1980s about a woman who finds a picture and/or a letter after her parents' death that leads her to a small town to investigate her past. Plot points include a big white house, a stolen ring (possibly emerald), a recurring image of a limp hand wearing said ring (which turns out to be a memory from the heroine's childhood of having seen someone pushed down the stairs and lying at the bottom), and the discovery that the heroine has a mentally challenged twin sister who regularly escapes from wherever she lives and returns to the attic of the big, white house.
I don't remember much about the male love interest, except that he has keys to the house, so he may have been a realtor or a handyman. (I was way more interested in the mystery than the romance, I guess!) I think the book cover was white, and I'm pretty sure it showed illustrations of the house and the ring on the hand.
If you can HaBO, I'd appreciate it.
There is nothing more amazing than a mystery twin, particularly one the heroine doesn't remember. I've read that device several times and it never fails to amaze me. Does anyone recall this book?