I was able to wake up a friend but I had a a lot of help. His wife had gotten dfd for mutiple acts of the big no no and he wasn't in the mood to go to many meetings. I at first had asked him a question that apparently set off alarms bells and so I had to step lightly. In time though I would just make small comments or a rebuttal to things he said, planting seeds. The fact that because he was missing meetings one of his elders came by and accused him of doing something wrong really helped me. I would just bring up little things from the bible showing how what they were doing was wrong and the scales soon fell off his eyes. Then I was able to show him more stuff from the bible showing how screwed up it is.
I think what had to be is the table set first by someone going through something that snapps them from thier delusion. Then one can introduce a new thought and the person is more receptive. You may ask your sister some probing questions about how she feels about being dfd and maybe bring up that it's really the husbands fault. Use emotion always if you can. Then if she's opens up you can insert the idea that Jesus would have not dfd you he was all about love forgiveness compassion etc. See where this takes you. If this works push the emotional thoughts some more planting seeds of doubt but don't push to hard. Let her open up and respond only a little to her thoughts. In time this may lead to you being able to share more.
Since you've been away from the cult you can Also maybe ask some questions and follow with a question like but I thought Jesus said this Or the bible said that. Again play on her emotions and stay in how she feels about things stay away from arguing over logical points or facts bring those up later when the door is open more.