“Is not reading the Bible a sin?”
As with so many answers to questions asked here, let’s say it depends on the situation. There have been times throughout history, both during and after the period when scripture was in the process of being written, when large numbers of its intended audience were illiterate, and not by their own choice. Literacy is a privilege and an opportunity not offered to all men and women, and surely the Lord would no more charge people who can’t read with the sin of not doing what they are unable to do than he would charge the innocent poor man for being poor.
Likewise, I don’t see him adding the charge of not reading the Bible to every one of the dead scheduled to appear before the great white throne, though it will certainly leave those who had opportunity to do so without excuse. But they have plenty on their plates without that.