likely not very diplomatic but I did try

Dear Cuz:
I’m sad to see that you repeat whatever you are told without actually checking it out or doing your own research. Atheism is the lack of a belief in god or gods. That’s it. The reasons atheists don’t believe in gods is that there is no evidence. Now, we do have evidence for the Big Bang Theory, the theory that your cute little sign gets so very wrong. It’s a shame that some people must lie about other people and what science actually says. The Big Bang Theory is the best and most accurate theory for what we see in the universe. A theory is what science uses in the scientific method. Scientists observe an effect and then hypothesize what caused this effect. They do experiments and more observations to confirm or deny their hypothesis. Then with they have a lot of supporting evidence for a hypothesis, it becomes a theory. Many things are theories, including the theory of gravity, which is also considered a law, which is when a theory invariably holds true, again like gravity.
You depend on the scientific method every day, when you use your car, a computer, modern medicine, etc. But when this scientific method shows your myths to be in error, you attack it without thinking.
Your little sign gets many things wrong. No, the big bang theory does not posit that there was “absolutely nothing”. There are other hypotheses out there that think there was indeed something, but we don’t know if they are right yet. We may never know, but that doesn’t mean that this prove that your god is the “right’ one or that it exists at all. The big bang theory also doesn’t think that anything “magically exploded for no reason”. Again, there are hypotheses for this too and again, no need for any gods.
Another thing it gets wrong is that no one claims that “everything magically rearranged itself”. The “everything” obeyed the laws of physics and chemistry. This sign, seems to assume a Dr. Seuss universe where things happen randomly. Happily for us, that doesn’t happen and it demonstrates your sign to be a lie. You depend on reality being the same everyday and it is. The theory of evolution, again, one more theory that has been shown to be accurate again and again, and one that you depend on when using antibiotics, also obeys the laws of physics and chemistry.
Now, you using this flawed sign gives me much reason to mock you since you’ve shown your ignorance, or laziness, to the world. I can also mock you and your hypocrisy for using the same science that supports such things when you find it convenient and comfortable but attack it ignorantly when you feel your religion is endangered. If you are a Christian, then you believe that a god created the universe; created two people who were ignorant of everything; created a “snake” and either allowed it in to the “garden” or didn’t notice it there; cursed the people for listening to the “snake” one of its supposed created creatures; flooded the earth completely leaving no evidence at all behind; gave people a list of laws not realizing that they wouldn’t follow them well; and finally decided to make a version of himself to kill to clean up the mess he created back with that “snake”. I’m sure you mock other religion’s creation stories, that magic cow in Norse mythology is pretty funny, so what makes your religion any better than theirs?