I've already admitted to knowing nothing about you. I'm just going with what's here.
Let's be real, here. You aren't perfect in your decision making either. You care her, right? In your own, sideways kinda way, I mean, she's your sister and you're doing what you're doing to protect her. Which would almost be an awesomely good thing. Except that it's ending someone else's life and hurting the very person you're trying to protect.