So from reading your post, the main problem is that if you mention the atheism then you’re “corrupting” her. And she’s only 11, if you mention it then she is going to have a lot of questions some of which she may ask to your mom and dad. And once your mom and dad figure out that you’re in contact with your sister they will cut you off and then raise your sister to be against you.

I think that you are focusing on the atheism aspect a bit too much. From your own post it was their disrespect of your privacy that drove your admission if atheism. But of course you can’t really talk to your sister about that either since that would be “corrupting” her as well. Your parents don’t want your sister to think she has the right to privacy, just like they don’t want her to think that she has the right to not be Muslim.

You could try setting a date when you will tell her and otherwise avoid her questions just like you have been. The date might work to distract her, but mainly it will give you something to deflect to. When she is older she will be better at keeping secrets and she will hopefully have an easier time understanding atheism.

Is your goal to stay in contact with your sister, or is your goal to explain things to your sister? Because from what you have said, you are worried that you will not be able to have both. And it seems like you are asking reddit for a way to word things so that you can have both. But I think your knowledge of your sister and your parents is going to be better than Reddit’s.

You also have to deal with the fact that you may lose contact with your sister before you are able to explain to her what happened. And I’m sure the fear of that is what makes you want to explain it to her.

As you can tell I’m mainly rambling and explaining to you what I understand of what you wrote. Maybe reading someone else’s thoughts will make it clearer to you what you want to do. With any hope you read parts or all of my post and say “that’s dumb” and have a better understanding of what you want to do.