My sister is my best friend, but what you talked about has been a real problem for us ever since I dropped Christianity. I only told her I don’t believe anymore pretty recently, so we’re still figuring out how to talk about religion and god (which is a very big deal for her) while we both disagree. It’s tricky, but we always figure stuff like this out. She’s quite open-minded, especially for a religious person, and that helps a ton. It’s still tricky though