I'm going to lie to my grandma about her 23andme test. Her mom wasn't her mom.
Why do you want to lie to your grandma?
I would rather keep her birth mom out of the picture.
Lying to your grandma about her heritage is a pretty immoral thing to do. She has spent part of her life living under one perception and it wasn't true. You not telling her is continuing that lie.
She's fine, she isn't bothered by it. Plus she will never find out.
How do you know she is fine with it, if you're worried to tell her it's because you know she will be upset. It is not your place to decide when someone should know the truth, tell your grandma.