How do I best support my sibling?

Hey all, so I'm straight, but my younger sibling (16, 10th grade) is non-binary and asexual. My mom, brother and I are the only ones that they've come out to, due to all of my family being either very Christian, old fashioned, or both. My mom practically dismissed it, and my brother is still young. It's difficult because my whole family lives in the same town so we see everyone ALL the time. Plus, the high school they go to is also filled with a ton of Christian or old fashioned or just stiffs with sticks up their butts. How can I best support them, dealing with all of this plus some anxiety and depression issues??