WHAT
THE FUCK
Your sister shouldn't be allowed to defend herself?!
What in the paternalistic misogyny is that fucking bullshit?
It's not even logical! If she learned magic or defense, she would be an asset to survivability!
But you "can't" tell her about the leveling system because you, specifically, "... have to be the one to protect her."
That is the single most retarded thing I have read in 2025.
In a manga, that is.