What to do?

[Angrboda]

The decision of how to raise your brother properly belongs to your parents, imo. And your duty is to support them in their decisions. If you can find it at all agreeable to your conscience, I'd suggest you continue to assist your parents in raising your brother in the manner they have chosen, provided it doesn't violate your conscience. Your parents' emphasis on your brother understanding is an important point, and underscores the need to respect the limitations on his understanding, as well as pointing out that the goal in raising your brother is his understanding of his world and himself. It might be argued that being incompletely honest about your feelings toward church for his benefit may not be the best way toward that goal, but that decision is out of your hands. I suggest you be as honest as you can be with him while respecting your parents wishes, and defer more substantial discussions of religion with your brother until you are older, and until he is sufficiently mature to understand the reasoning behind your actions, not just the surface.