Hi exbrahmin
You are not the first to ask this question on this forum. You will probably get a lot of advice thrown at you here, some will make more sense than others. One thing that I know for sure - people become BK s for many reasons, but mainly it is the experience had, or the hope of the experience, in meditation/Yoga. The rest follows. They remain BKs for different reasons. If you read through the topic - How did we fool ourselves into becoming BKs - you may get a broader perspective too.
You cannot just remove someone against their will of course. But if alternative and equally strong experiences can replace those, then the person can naturally leave it all behind. Actual distancing from the BKs alone can help, but is not enough - also needed is regular healthy company with others, and alternative activities that also offer depth and learning of the inner life and the emotional life (which she is now exploring).
I would suggest looking to encourage her to spend time on things she has always had a deep love and interest in, to build on those. If she can have real experiences, there is less likelihood of using the BKs as a substitution (e.g. you know they actively encourage the transference of emotions like love, to Baba, so encourage social activity that lets her love real people, be with people she loves, and doing things she loves. Raja Yoga may then just remain a fringe activity which falls away as Life's richness opens up for her).
You need to comprehend what her attraction is, and her character. Many of us here on the site are intellectually and philosophically inclined, but many people are not. They respond emotionally, so words don't cut through. They can become defensive and don't listen.
If your Sister is intellectually driven, then facts and figures and argument may make her think again. If not, she will close up and not listen at all. But even then, intellectual people are often the best at justifying anything to themselves. But if she is a mature adult her journey is now her own. You can help ensure she doesn't lose perspective, and as you are ex-BK let her know it is not a closed world, and exit is always possible.