Hi Ceemee, To use something from AA and Al-anon, your sister has her own higher power, her own path, and her own life lessons that she needs to learn. In all probabability, she will be ok. She seems to me to be taking care of herself in the best way she knows how at this time. Distancing from an abusive family situation is very normal in many stages of healing. Not having been raised in an organized religion, I don't have a prejudice either way about how good as people thel the paternall pastor and therapist wife may be, but they may be helping your sister reparent herself. If it all blows up, your sister has survived an abusive childhood where she didn't get her needs met and will probably survive this too, maybe even further along on the journey to healing.
I happen to think that the best thing we can do as siblings, as friends, as parents, and as spouses, is to let those whom we love know that we love them and that we respect them enough to let them have their own struggles and find their own answers. To me it is all about respect and boundaries. I can't give somebody my experience. I can share who I am, but everyone who is searching for emotional healing has his or her own process and time-table.
For me, that means shedding the old roles of birthorder, like "the big sister" or "the decisionmaker." I can relate to you because I am the oldest sibling and have some oldest child characteristics that can just come to the fore if I don't pay attention. If I am not aware that they are not appropriate, they can can wreck every day situations that don't require a decisionmaker...who needs a decisionmaker at a party, for instance? On the other hand, in an emergency, having that attribute can be helpful, if, and only if, I am the one there that is really qualified to make decisions. We can drag this birth order stuff around, sometimes unaware, until we have an epiphany and decide to stop with it already.