So one of the pages at the end of Chapter 1 gave a quick bio of Youko (the black haired one with a kimono) and said that she is the main characters blood related older sister yet she calls him Nii-san yet he calls her Youko and not Nee-san or anything. Mind you I only read chapter 1 and this might be explained? Or is it just a translation error?

With no spoilers (hahah as if I could spoil, you need translation scripts to spoil 😛 😔 🙁 ), it's not a tl error, I double checked that stuff myself (I've figured out enough japanese to personally proof the names/suffixes in the translations).

It probably has something to do with his opinion of her -- it's not said outright so far, but I don't think he likes her very much. No idea why, although I have some theories from guesswork and what's been implied so far.

Now it's really bothering me. Is the author trying to troll us or did he miswrite? I just read the most recent scan, chapter 5 and Youko's classmate acknowledge that Youko was the younger sister. WTF is going on?

You must be very slow at many things...