To the OP–talk to your parents and tell them how you feel, and see how THEY feel.
It’s possible that they feel the same way as you do. OR–it’s possible that they really don’t have any objection to your brother’s decisions. If that’s the case, it’s sad, but don’t argue with them. You can’t change their mind. Some parents believe that they have no right to tell their older children what to do. Sigh.
What my husband and I would have done is to contact the girl and ask her to join us for coffee at a local coffee shop (or whatever she likes–the idea is to meet outside our home, and not at her home). Once we were seated together, we would have told her how much we like her and how we are pleased that our son has found someone like her to date. THEN…we would tell her how important church is to us and how we miss the whole family going to church, and we would ask her to please talk to our son and help him to go to church and GO WITH HIM so that they can be “spiritual” together and be a couple together in spiritual things, not just this world.
I would make it sound really cool and spiritual for the two of them to be at church.
And…we would make sure to tell her that there are many different Mass times, and if they don’t want to go to a Sunday morning Mass (I don’t like going to Sunday morning Mass!), that they can go in the evening, or late morning, or whenever the parish offers Mass. I would also explain that there are “contemporary” Masses if they prefer a different kind of music, and I would do my research in advance and be prepared to tell her where in the city (if not at our parish) those Masses are held.
And I would thank her so much for helping our son to grow up to be a good man and tell her that is she ever needs anything from us to please come to us.
That’s what we would do–recruit his girlfriend to help us, but make it clear that we want HER to be his spiritual friend, and that we won’t force them to do this. It’s their choice.
If she is truly sweet, this might work. If she is a druggie or…well, you know–sigh.
One thing that will NOT work is to force him to abandon her and go to church with the family. This will only drive him away and into her arms., and it will create resentment in him that will last the rest of his life.
But to do nothing and hope that he and she start going to church gives them both the impression that it’s not really important. Hopefully the parents of this young man will try our approach–make friends with the girlfriend.