At first this was structured a bit like a romance novel. (All character names are speculative at best, probably wrong.) Our heroine, Kate (maybe a redhead), has just met and married a rich young man (whom I'll call Lucius). She moves into his family mansion, where he lives with his mom and her sister Lavinia. Lavinia and Mom are identical twins with long dark hair. Unbeknownst to everyone, Lavinia is a witch. In one scene, she goes to a coven meeting in the woods and sleeps with Satan (whose body is described as ice cold). During the meeting they might have sacrificed a young female virgin, possibly by killing her on a stone altar.
Lavinia also has the hots for her nephew, and she hates Kate. Somehow she sends Kate back in time a couple hundred years, maybe to Elizabethan England or thereabouts. While Kate is back there, she unwillingly becomes a prostitute. (In one scene afterwards, some men try to rape her beside the road but she's like "Oh no boys, you can't afford me" and they don't do it.) And Lavinia takes advantage of her absence to seduce Lucius. I think it has to do with either her pretending to be his mom, or his mom dying, possibly both.
Somehow Kate comes back to the present (I think she hooks up with some "good witches" in the past), tells everyone about Lavinia, and she and Lucius live happily ever after. Except, in the last few paragraphs, we learn that Lavinia is pregnant with Lucius's son, and furthermore, she was really Lucius's biological mother all along, so she's happy because she can start the whole thing over in a couple decades...
I should add I read the thing in the 80's sometime, and it seemed to me to have been written in the 60's or 70's. Pretty sure the setting was somewhere in the British Isles, and not sure of the time period (but I'd wager on sometime in the early 20th century -- I feel like people had cars and phones but no TV or radio).
Also I posted this on livejournal (http://whatwasthatbook.livejournal.com/2421376.html) and another commenter said:
- "I think it was probably written in the 70s."
- The aunt was trying to marry the father for his money
- "The heroine made a pact with her coven to give up her first born child in exchange for their help. I don't think the aunt was pregnant when she left."
- "There was a sequel too! It wasn't as good, but it continued with the story. The heroine had had a daughter that the aunt kidnapped as a baby and raised as her own. She'd run away to another city and married a rich man. The sequel ended with a cliffhanger with the daughter kidnapped by pirates I think - she was definitely kidnapped by boat. I have no idea if there was ever a third book or not."