Lying to people you love is never the right thing to do. When you lie about who you are, you robbing yourself and your family of a chance to truly know and love each other. If your lack of belief is more important than the fact you're her child, she never loved you only who she wanted you to be.

Humor an old lady. She probably senses your apostasy and it's her way of nudging you in the right direction and/or reassuring herself that you're at least covertly religious and therefore okay. It's a pretty low-cost way of making her final years less uncomfortable. Is it technically enabling? I guess so. But the alternative is even worse, most likely. Arguably, the alternative is cruel.

I have an elderly cousin who is being a real asshat right now on social media, full-blown Trumpism and xenophobic expressions of Christian fundamentalism abound. Reading between the lines, the reason she's so activated right now is that a beloved aunt died and she's feeling more lonely and afraid of her mortality than usual so it's a dysfunctional way of making connections (even negative ones) and also of trying to set things right in the world.

Since she's inflicting this mostly on other people I haven't intervened, but on my own FB account, I did thank her for letting me know of our aunt's death and offered my condolences and a couple of appropriate reminisces. I could do this in spite of the fact she's a needy, self-absorbed religious nutter, even by the standards of my former evangelicalism. Even though she's a Los Angeles resident who once got wind that I had been all the way down in San Diego on business and then complained to the rest of the family that I didn't drive six plus hours round trip north to visit her ... someone I hadn't seen in person since I was a child of 13.

People can be immature, irritating, and rude, and still be humans, after all. And so it costs me nothing to be empathetic and compassionate, even to them.

Don't tell her.There will be nothing gained except to make her unhappy. What she will get from this is she won't see her baby in heaven one day. She doesn't need to carry that with her for the rest of her days.