I've heard of a strategy that can help some people challenge their belief for issues like homeopathy or flat earth. The idea is to ask them to rate on a scale of 1-10 the strength of their belief. As in, "how convinced are you on a scale of 1-10 that the earth is flat?". If they give anything less than 10, you then follow up by asking them why that score and why not a higher or lower score as in (if they said e.g., 8/10): "What would make it a 9/10 or a 10/10?".

Basically having them "ironman" the opposite side of the argument can cause people to start to think more objectively about the issue. However, if the person says 10/10 there apparently isn't much that can be done.

I've never tried this myself as I don't have any flat earther or homeopathy believers in my entourage, but it does seem to make sense and I've used this to question some of my beliefs myself. Not saying it would work with your family members either though.