I am also a mother of a 12 year old and left my somewhat repressive religion last year. A dear friend recommended this book and it has been a game changer. I would suggest you read it as it lets you know what situations are perfectly normal and how to appropriately intervene when they are not. Best of luck to you, Momma. Tween years are no joke!
Untangled: Guiding Teenage Girls Through the Seven Transitions into Adulthood https://www.amazon.com/dp/0553393073/ref=cm_sw_r_cp_api_i_0X49V4A0HV4TXJM449F8
I didn't read your full post. But from what I read, you should consider reading (or listening) to the book Untangled. I just listened to it and it was tremendously helpful as a parent of a 6th grade/11 year old daughter.
https://www.amazon.com/dp/0553393073/ref=cm_sw_r_cp_api_K4elzb52PM78C
I would recommend getting this book and all of you reading it (including your sister). It helped me tremendously understand where my teen was coming from and my teen read it so she'd understand what was happening to her: https://www.amazon.com/Untangled-Guiding-Teenage-Transitions-Adulthood/dp/0553393073
Also, as far as your sister being a people pleaser and gullible, this might be a self-esteem issue (she's craving acceptance and external validation). That can be worked out with a good therapist. She can also try this here: www.innerpathways.org (the one for teens).
Your parents might need to be open to readjusting their parenting style as well. You can't parent a teen the way you parented an 8-year-old or a toddler. So your parenting style has to adjust and evolve as your child grows older.
Good luck! I hope you all find the help you need. <3
I highly recommend this book:
https://www.amazon.com/Untangled-Guiding-Teenage-Transitions-Adulthood/dp/0553393073