Why does it matter? No need to overcomplicate things. The fact of the matter is Sheila was religious to an extent. Plus she had delusions all of a religious theme as a result of her mental illness and those symptoms would remain for as long as the illness that is causing them.
IMO taking the delusions aside Sheila seems like someone who would be religious when it suits them and when they need faith for what ever reason, Unlike June who was a 24/7 fanatic
I don't recall that any of her friends has said of her that Sheila spent any of her time with them talking about religion but as I understand how far she may have gone to raise herself in June's eyes, she MAY have used religion as a way of doing it which strikes more of conscience decision than delusional behaviour.
I believe you read June wrongly. I don't believe she was a 24/7 fanatic. I DO believe she was a dedicated and devout Christian who turned to her faith to put right those things she believed were wrong within her children.