Me thinks your Sister is reasonably correct, my problem with it, that is a very big knife! A double edged 4" Blade would be easier to use, and retain.
An old kit bag, well wrapped in duct tape, great for hanging in the garage, and use to practice on. Might change her mind
I would not use this idea, if I had to shoot one handed, I could, 40 years of bulls eye shooting has me real comfy with the one hand gun. But two handed is better, quicker reset after first, and multiple shots, more accurate too.
You can always go to one hand, but the off hand empty give you door opening, and blocking uses. Just an old guys opinion.