Hi Bttrfly972,
Pretty gut-wrenching stuff. If you have the fortitude to write it, you are sure to have an audience. A comment: You must make each statement by a new person a paragraph by itself. I know that makes for a lot of one sentence paragraphs, but that is the nature of dialogue. About the knife: I would have had Katie discover a hiding place by accident, like knocking off the head of the bedpost in a fit on angst and discovering that it is hollow. The knife could be her mother's favorite paring knife, a perfect fit for Katie's little hand. Maybe: <Mother looked everywhere for it, but not in Katie's bedpost. And when they were away, and Katie was alone, she crept into the kitchn and got out the knife sharpener. Sharpening, sharpening, for the day when Katie the Brave would protect Katie the Helpless.> You can't really sharpen a serrated knife as far as I know, though.
Instead of stepping back to admire her work, I would have Katies' heart pounding, terrified that Luke's bloody corpse would rise up and get her. For a young girl to reach this point, she'd have to be pushed way past ordinary limits. Maybe: <She stepped back, grinning, for her terror was ended, yet surely the evil that was Luke could not die so easily.> A young person might not be secure enough to want to leave her home, especially if an abuser kept her isolated and confined, which I'd expect that type of person to do. At this point, she would be so traumatized not only by the years of abuse, but by her own horrific act, that I would expect her to do something really weird or stupid, like chopping them up until her rage was spent, filling the cat food dishes with their remains, then hiding under her covers.
Lots of interesting possibilities with this story. Best luck.
Rebecca Anne