I carry a knife with me on a daily basis, and one stays on my nightstand; however, if I hear a bump in the night I grab my firearm, and in my weak hand I grab a flashlight.
I have had knife training, and every knife instructor will tell you that the object of coming out of a knife fight is to be bleeding less than your opponent as opposed to not bleeding at all.
I do not recommend people put a knife as a defensive tool unless they actually know how to use it from getting training in that area. A knife is rarely an instantaneously incapacitating weapon. A crowbar in the hands of a criminal is a much more formidable weapon than a 10" knife in the hands of a person who has no idea how to use it defensively.
IMO, your sister should leave the knife away, and either use her weak hand to get a proper two-handed grip on her firearm, or use it to hold a flashlight.