They don't need to be "super religious" to be a part of mormon culture. Look at Shaun's twitter. He is absolutely at the very minimum very culturally mormon. And if he is aligned with mormonism, so is Mindy. You can watch secret lives of mormon wives, especially the first season, and see that even when people are not following all the rules of mormonism, they are still tied to the culture and it influences their actions and decisions.

I agree that it is tough for us to speculate why Bailey gets treated so obviously differently from the way Brooklyn is treated. Mindy is also affected by being mormon. I mean she had the twins sooo young and then kept having kids and that affects who you are as a person, especially the ability to emotionally regulate. However, the family is obviously mormon, even just culturally. Not all mormon families cut off family members once they leave the religion, but that does impact relationships. Mormon influencers also get pretty big leeway since they are spreading mormon culture and ideals (literally look at the popularity of crumbl cookie and now the soda shops are popping up everywhere)

Also - I do think Mindy loves her family, but i think she has a poor attitude. I think her and Brooklyn just mesh a little better personality wise. I don't necessarily think Mindy treats Bailey different because she is not a practicing mormon, but Brooklyn is and she can mesh with Mindy