They did not meet when they were 5. They met when they were 10. They're 14-15 now, so only a few years has passed. It explicitly states so in both the manga and novels. When he was 5, was when he sent his mother to jail---which is probably what you were thinking. But they met each other at 10 (as in OG Mai and Kurobe).
The current Mai is a transmigrator, who woke up at the beginning of HS, which is the start of the series, so she's known him for a few months at best. About 340-ish days will pass before D-day, so she's spent a total of almost a year with him at best.
i read it here. https://chococats.home.blog/projects/desuge-imouto-series/
yeah my bad, forgot the timeline a bit
still i thought that she sort of got the memories of the og Mai? so if she was 9 and now is like 15, they’ve been sibs for around 6 years and he was acting like a carrying brother nothing more… weird. i might be wrong on that too though
and if she doesn’t have the memories of the og, it goes weird in a different way like you know he loves torturing animals? and is going to become a mass murderer? and is going to kill you? she wasn’t sure if her plan was working and only focused on preventing it all from happening, for her he is still the same as he was and yet now she’s in love with him?
so yeah for me it’s insanely weird either way
Hmm...It's been a century since I read the novel but I think it's less; more like 5 years.
(School terms start in spring, so Mai would be 14, Kurobe 15).
If I recall; I think the original Mai had a crush on him/was a bro-con, and the current Mai thinks those feelings are residual from the previous owner. She doesn't immediately crush on him, but she does start to take notice of him around the time he spent with Himegasaki; but she's extra obtuse to her own feelings. It was just a lot more obvious in the novel that she did like him but was unaware/in denial.
---But yeah anyway, you're still very right. He's still a sociopath lol. She's a bit of a weirdo for liking him regardless; but they're both weird and into each other so it's what it is.

so he confessed that he is SEXUALLY attracted to her and she just accepts it like this??? like it’s nothing? sis he was your bro since you were five, don’t you have any sort of moral sense or wall that doesn’t let you see him as any other dude you could potentially date?? i loved this so far and were excited to see how it goes, cuz i didn’t think they would get a happy ending BECAUSE of Mai’s inability to see him in a romantic way
gess i was wrong (● ̄(エ) ̄●)