Things are starting to piece together. The reason maybe why past Shi-chan was very bratty was probably because of Akio's actions. That "I hate you" was probably the reply from Akio to Shiori at that time and probably really hurt her (it was the first thing she remembered after losing her memories afterall), probably since he was scared that the family would break apart. This would mean she started playing with other guys maybe as some sort of protest or to get Akio jealous? It also explains why she snapped at Mizuki during that hospital scene after their father's death, she was (probably still is) a huge brocon lol.
I doubt Kai was fine with that revelation either, while it looked like he didn't have any problems with it and that he's pretty easy to read. He did change his face when they were hugging (so Shiori won't see). It's not too far fetch for him to be affected either as he did know the Shiori did once love Akio not as a brother, this happened to Mizuki as well when that was revealed to her. We might see a repeat of events, thought not maybe as bad as Mizuki's.