I am seeking the title of a book I read some time ago and have forgotten the author as well. It's about twin sisters, where one twin (who's married) asks the other twin, (who lives in a large city like NY maybe)  to stand in for her, without much of an explanation about why she wants her to do this. The twin agrees and goes to live at her sister's home and pose as the man's wife. He's apparently quite wealthy, lives on a modern ranch, and I think also has one or two children. There might be other relatives living in the house as well. At first everyone accepts her, but then they start questioning who she really is, because she's much kinder/sweeter than the real twin sister who lived there. Later in the book, I think there's an attempted murder of the twin sister posing as the rancher's wife. I'm just not sure if I'm remembering this correclty. I've read The Switch and Thursday's Child by Sandra Brown and it's not either one of those books. Does this sound familiar to anyone?

Could it be Double Take by Brenda Joyce ? It's been a few years since I read it but I remember it being similar to what you are describing.