I think it’s because the fact when she came into the world she heard “if you let the novel play out you go back” so she thinks she did something wrong instead
I don't think they are brother its more like he may have a little brother because he lost his memory and the little brother or him is the guy childhood friend/ first love (maybe ┑( ̄Д  ̄)┍ its a guess)
Is he stupid ( ̄∇ ̄")