You know... I get it. It's not exactly easy to suddenly just kill your little sister.
Like, even if my little sister turned out to be like Luna, I would STILL find myself hesitating when the chance comes up to let her die. Because no matter how much she betrayed me, I would still think back to when I first got my little sister and would find it to be unbearably painful to just let her die.
Even if my sister didn't share the same sentiments as me, that's just how I am.
Thankfully, my sister is nothing like Luna and I have complete pride in her.