What's it going to take for the "tipping point" to be reached, so that closeted Atheists, such as myself, can come out to highly-religious family members? 1/5 is quite a bit.
My reply to this would be to just come out already. A relationship with your family, where you pretend you believe like them, is not a fulfilling relationship, and there is little point to it. Even if you think you love your parents/siblings, going along with this lie makes you unhappy because you cannot be yourself. So you're trapped between keeping them happy, and being happy yourself. Unfortunately, this is an either/or situation. No in-between.
I came out, and my dad and I haven't had a meaningful conversation in more than 15 years. I wouldn't say he hates me, or anything so drastic, but every conversation is strained and we stick to superficial topics. Mothers will be mothers, they forgive and move on. You'll always be their child. My brother is openly antagonistic towards me, but with him I can stand my ground as we're on a level footing, and brothers don't really keep grudges for too long. I am as antagonistic towards his beliefs as he is towards my lack of it. I also have a sister, but she's never had an original though by herself, so that's that.
Just come out already. At the moment you fear the unknown. Once you're out, the chips will fall where they will. Those that love you truly, would do so even if you think differently to them. And those who turn against you were never worth cultivating relationships with to begin with.
This is a hard decision, but only decisive action will set you free, and allow you to move forward. The pain fades over time, but never really goes away. However, these are sacrifices our generation of atheists make so the next generation has an easier time.
Keep strong, and best of luck.
If not you, who? If not now, when?