No one is asking the OP to do anything other than attend an event where her nephew is singing. Either the venue is offensive to her, or it is not. If she later attends a wedding at the church I would find her mitivation for not attending her nephew's event questionable.
My grandmother would not attend my wedding because it was not religious. She would not attend my brothers wedding because it was "the wrong religion." Are her motivations questionable when she attends other events with us, secular Easter dinner, secular gift exchange on Christmas, our meet the baby party (which happened at the same time most of the family would have had a baptism or baby blessing."
Actually, to me, yes, they would be. Even if it were my grandmother.
That's an opinion or judgment that I am entitled to make.
is entitled to decide that my negative opinion or judgment on this issue doesn't really matter to her, and to continue to do what she wants in this regard.
We're entitled to disagree with someone and even to make a negative judgment about them based on the evidence and our own ideas of appropriateness, etc., etc. We're entitled to let that influence how close our relationships
with them are. After all, we do not share values, and that *always* influences relationships
And they're entitled to disagree with us, and to ignore our negative judgment.
I didn't see TurtleDove's comment as one that means, "You need to change," but more as one that means, "You need to be aware that people may react this way."