I was sort of in the same position as your sister at that age. Extremely religious parents who were pretty controlling and who found out I was sexually active. It wasn't fun.
I guess my big question would be --what are her plans? In my case, I definitely wanted to go to college, and my parents were paying for it. It was so worth it to me to just shut up and pretend to comply for 8 months or so until I knew I'd be off to college and could do what I wanted to do.
If I was planning on moving out at 18 and getting a job, I wouldn't have been so compliant.
Her plans are similar to yours, go to college which my parents are willing to pay for. Financial support is a big big reason there is this struggle. I'm relatively certain if they were not going to be putting her through college she would have just left at the start of this trouble. So right or wrong it's probably in her best interest for the future to try to get along to make it easier for her to get the education she wants.