K.I.
As one stepmom to another, you are doing a great job! You will get no slamming from me either!
This is a tricky situation. I also agree with your thoughts on setting her up with a counselor, she will benefit by being able to talk to a neutral 3rd party.
Part of me thinks that you could inform her of her ability to ask a judge to end her visitation, but she *must* know that that would require her to stand up in a court of law and proclaim to the world (and her Mother) that she no longer wishes to visit her Mom...which is a hard and traumatic thing for any one to do, let alone a teenage girl. You might want to float this idea by dad and see if giving her this *option* might, just might squash any ideas she has about doing so. When laid out like this, she might just see the error in going this route. It is very dramatic and seems very final, ya know?
Knowing myself, I am pretty sure I would (if she were my stepdaughter) reassure her that she is NOT a bad person by choosing, at this stage in her life, to not follow any one religion. I would however encourage her to play 'lip service' to her mother for the time being. She is only a mere 3 short years away from being 18y/o and an adult that no longer HAS to go to required weekend visits. For the sake of keeping the peace, it might just be worth her while to 'suck it up' so to speak and go to church/temple/whatever they call it, while at Mom's house and be done with it.
Her only other course of action is to be truthful with her Mom, for her to have a sit down conversation with her Mom and explain that she is putting to much pressure on her and that it is doing harm to have to listen to the 'you are being a bad person' lectures every time she comes over. She could tell her Mom that she really trying to have a good relationship with her and enjoy their time together but in all honesty that she, the Mom is making it very difficult...and quite frankly it is making her NOT want to come over. Maybe the Mom will back off a bit? heck, maybe she will be mean and tell her she doesn't have to come anymore if that is how she feels? Who knows?
~I can 100% put myself in your shoes. My husband's Ex is Mormon and my stepsons have had to listen to their Mom bash their father and I (and even my in-laws) b/c we are not 'righteous' enough and how we have all been given the opportunity to accept the 'one true' religion and how we have not chosen the correct path and how we will NOT be in heaven with them. This is a harsh things for lil' kids to hear. My SS's are now 21 and 18 and neither of them have chosen to go on missions and follow their Mother's religion and I believe whole-heartedly it is straight up a direct result of their Mother's pushing and bad mouthing all these years...b/c only god knows that both their father and I have said NOTHING negative about their Mom or their religion...other than it is 'not for us'.
Hard spot you are in...but you are doing great and will figure it out. As they say...this too shall pass! I know it is a cliche...but it is one for a reason, b/c it's totally true. Honesty is my best and only policy. Talk with your SD and let her know that this might seem like a HUGE deal right now....but that she WILL look back on this, sooner rather than later and realize that it is going to be OK. When you are a teen *everything* seems like a 'life or death' situation...the best thing you can teach your SD is that sometimes you just need to take a step back and breath and go with the flow until things pass!
Eventually your SD and her Mother will have to have a frank conversation about religion...and your SD will have to speak her truth, no matter how much it might hurt her Mother....and it is up to her Mother to get over it. Period. End of story.