I've always heard that you can't convince someone against their will, and it seems your bf's sister has her mind made up. I'm sorry about that.
Having said that, however, I intuit that you don't want to give up the "fine fight" for your bf's sister. In this you might want to consider choosing your battles carefully. So along with the others, I suggest the trinity perhaps isn't the battle you're looking for.
I personally think that religion is an emotional matter, and you might want to consider discovering what about your bf's sister's emotional makeup the JWs appeal to and work on that component of her interest in them. She might even be willing to disclose what so attracts her if you ask in the right way. Like a straightforward "what is it about what you're studying that attracts you so much?" At least then you'll have a place to start.
Eric Hoffer posits that there are known personality types that are attracted to cults. You might want to get his little book "The True Believer" and see if it gives you any clues or insights about what's motivating this person. Can't hurt. And it's better than the shotgun approach of attempting to devalue everything she's being told. That approach likely makes you just appear negative. Perhaps there's a positive approach you might use? I hope so.
Best of luck to you.
Francois
My $0.02