Honestly, I'm really enjoying this "isekai", the slow pace is very refreshing, most isekai at the beginning of the story hit you with a lot of info and don't even explain things in a thorough way.
And I agree with the rest, if we look it from the imouto's perspective, his brother is having a psychotic episode after the "transportation to another world" event.
Chapters should be longer, only 10 pages in an irregular way makes it seem that they're short, especially since most of them ends with a cliffhanger.