River Rising by Dorothy Garlock
April Asbury comes to Fertile, Missouri, to work for the town doctor, Todd Forbes. On her way into town, her car gets stuck in the mud during a rainstorm and she has to walk for help. She ends up on the doorstep of Joe Jones, who falls instantly in love with the young nurse. Meanwhile, the widow of Ron Poole, who was killed after raping several women in town, has found papers documenting all the children whom Ron fathered, including names and birthdates. The widow Poole sets out to prove that Joy Jones isn't Joe's younger sister but his niece, the product of the rape of one of Joe's sisters.
Suspecting that the widow isn't mentally stable April tries to get some help for her but also finds herself caught between the affections of both Todd Forbes and Joe Jones and eventually has to make a choice.