I am confused.
On the one hand, you say you are still a Christian.
On the other hand, you are thinking about coming out as a non-theist.
On the third hand, you don't like the specific church your family goes to apparently because they won't like you coming out as ... a Christian who is a non-theist?
I am wondering whether you can find an trustworthy adult you could talk with to sort out your feelings and thoughts. I wish that trustworthy adult could be your parents.
How did your parents handle your sister coming out as gay? If they handled it well, then maybe they will handle your revelation well too.
If you do decide to talk to your parents, instead of just blurting out that you are a non-theist or act defiant, ask them with a sad expression and a confused look for help in sorting out your feelings. This posting tells a very believable take of your confusion. Tell them this story. (As I write this, I may sound like I am advising you to manipulate your parents. This is not my intention. I am suggesting how to broach this sensitive topic so they will listen and help because this is what you need.)
If your parents didn't handle your sister's coming out well, could you go with some friends to another church while you sort out your feelings and thoughts? Tell your family you would like to explore other churches. Don't lie to your parents claiming you are going to church with friends when you are not. They would eventually find out and this would make you appear untrustworthy (as well as being just wrong).
Good luck.
Really good luck.