1. It sounds like a cult, calling itself a religion, using 'Jesus' as a cloak to push their real agenda. If it were a religion, there would be some mention about god giving human's free will.
2. Tell her you prefer to be called by your birth name and find it uncomfortable to be referred to as 'sister'. Notice what her response is. If all she give you in conversation, is repetitive, or chant-like answers, she's stopped thinking for herself. Something has happened that made her give up the will to freely think and choose.
3. There are many dangerous cults (they call themselves religions) that are basically into mind control, or wayward practices that can injure or kill humans. Examples of dangerous practices: not getting treatment for serious illness, refusing a blood transfusion that could save a life, playing with lethal biting snakes during the service, removing male genitalia — remember the Hale-Bopp people?, fasting — denies nourishment to the body, so that the body looks to other body muscles for nourishment, etc.
4. Point is, drastic changes in personality are big Red Flags that something is seriously wrong. Talk to your parents, and your physician about it. A physician may be able to recommend appropriate help for her.
5. There's nothing wrong with believing in something, but when behavior becomes extremely bizarre, robotic, listless, or possibly dangerous, you have to take it seriously.