she's traumatized dude, in the school incident less than 20 students survived. it's implying that because she's already lost a ton of friends, she doesn't want to lose her brother as well.
Just like whatintheBL said she's not ignorant but rather afraid to lose someone important again after experiencing such horrible thing. I think in the emotional state that she is now, I might also say the same thing as her after all she only have 2 family member left.
Still it doesn't change anything for me. Her fear of losing someone doesn't equal a death of tons of people ¯(◉‿◉)/¯
She doesn’t know or really care about those ppl and she loves her brother so it’s obvious that she doesn’t want her brother to die
Aaaaa why "don't go oppa" ??? Don't go my ass he has to save Japan and a lot of people, how can his sister be so ignorant?