This is an LN promotion manga work if the MC ended up meeting the "sister" first and decided to stick with her in the game world. Apart from that there's nothing special here, can't say much on behalf of 19 chapters other than it just being a preview of things went differently? Leon just goes his way navigating through the game world with his "sister" while the focus on the other characters are next to none. The art style is relatively different but the storysetting is basically the same. Perhaps that's just how iconic the main story line is, when you read other versions like this andreact calmly?