Thank you OP. Your poor husband. I can't imagine what it must have been like for him and his siblings. Abuse, religious fundamentalism of any kind, control - they've endured it all. He has been incredibly brave under incredible pressure. And so have you. Well done for keeping your head and staying strong, letting him come to his own realisation.
Keep up that I love you, I support you messaging - remember 'for better for worse' isn't just a romantic piece of wedding fluff.
I'm also so sorry for his sister ... But if she is allowed in, it will be the thing end of the wedge, a crack in the door. She isn't your problem.
If she'd been allowed in, she would have been influencing your children from within, which may have been the goal all along. To get them back in, from the mother who kept them out. Work on her brother from within.
I referred to a 'commune' earlier on - but your update suggests that could even be part of the plan.
Please please help him to every kind of therapy. If you Google therapy escape religious cult etc you'll find a lot of articles, advise and resources to share with him. Keep up your approach, that he must seek therapy, for a healthy future for your family.
Be prepared. For attempts at getting to him. New social media accounts, letters and parcels, different phone numbers, unexpected visits and arrivals. For rumours and reputation damage attempts. Is the religion itself; leaders, other members etc, likely to try to cause trouble as an organisation?
It may help him to write his memoirs, set down what happened, what this religion really is.
All the very best to you both and your family, you are the future.