Romance gone wrong. Two sisters in love with the same guy.

[Megan]

a romance that takes place in Texas, two sisters adopted by their grandmother, both fall in love with the same man. The older sister cheats on her husband with her birth father, is murdered. Husband leaves. Other sister stays, changes, suffers from insomnia, loves horses, brings husband back for her grandmother

[Aerulan]

Shades of Twilight by Linda Howard, they're cousins but I'm pretty sure since everything else fits.

[Kris]

Megan, is this an adult contemporary romance?

What are the main characters' jobs?

Around what year did you read this book? A range of years is fine.

[Kris]

Here's the Google Books preview of Shades of Twilight by Linda Howard - Aerulan's suggestion:

https://books.google.com/books?id=Wfe... There are different cover images.