I have been a professional theatre actor (don’t really do this professionally any more) and dabbled in tv/film. I also have Christian friends in the profession. Training is big in this industry. It shows not only that you are committed, but also gives CDs reference to your ability, background, and a person who can vouch for you (the teacher). A resume with crummy or no training will be passed over by legitimate agents and CDs.
I think your sister should study something that interests her in college, but also that is very practical. She could then minor or double major in theatre/acting. If she lives in NYC or LA, she could also study with one of the legit and better acting teachers, though doing both college and studying acting would be hard to do at the same time (but it’s possible). I think it’s okay to pursue music/acting when you’re young and don’t have family responsibilities, but she should go to college first since she’ll be new to the industry. But she should finish and not languish there (i.e. don’t waste time).
As far as the perceived sliminess of the industry, it’s overstated in general. Most actors are very nice, even shy, people. They are not normally the problem. The industry people normally just want to make money (that’s where their success/job security lies) and casting notices are very upfront about anything like nudity, working in water/heights, sex scenes, etc. No one wants filming/rehearsals to start and then an actor pulls out of the project. So being upfront is standard.
Here’s where the true sliminess is normally: the fringes. People who talk big, who act like they’ve got something going when they don’t, who are looking for the gullible/naive people to take advantage of. Your sister must demand professionalism from herself and all others. Legitimate jobs are not found on Craigslist (ever), legit jobs have contracts/work memos, legit jobs have a crew and a schedule, legit jobs can be non-paying, but they do not cast or film from people’s apartments/houses. If your sister has standards, she’ll be fine. If she can’t get legit work, then she should know that no shady source is going to lead to it. It just isn’t in the cards for her.
One final note, Christians and other religious people do work in the industry. They are blessings to their coworkers and the industry as a whole. They may have opportunities that they turn down from time to time, but no one will hold this against them. Every actor has to make those choices for themselves and they can be religious or just personal tastes/comforts. Sometimes God calls people to places to be a light and to show his love.