I'll be honest, I can't find myself getting mad at any of them. Cause they are written so badly that I can't see them as real believable people.
I mostly feel sorry for the little sister, because she's oblivious to the fact that MC likes her (because she only has eyes for his brother)
and that the brother doesn't actually care about her (because he only has eyes
her sister). But she's apparently also kinda just simple-brained at least according to the chapters of the WN that have been covered by the manga thus far.
The MC's a stupid idiot but that's just because he suffers from Male RomCom Protagonist Syndrome, and the brother's astupid idiot who comes off like the aggressive & domineering bad guy because the MC's scared of him because "he's so high-spec compared to me" & the MC's crush happens to like him, but is also just uselessly pining for the one girl in the entire setting who isn't lusting after him.
Kano's probably the most at-fault because even though she likes the MC she refuses to come out and say it, making her just as immature as the others who are all at least a couple years younger than her, but....the brother can't take the hint that she doesn't like
him, and I guess she lowkey
hates her sister because the MC is hung up on her, even though Kano feels terrible for feeling that way about her sister - according to the WN anyway.
But otherwise this is one of the sillier premises for a love quadrangle I've seen in recent times, agreed.