Fuck it. Aftons and their knives meta.
William: As we know, he uses kitchen knives. Made for cutting meat, preparing food. Not made to hurt people, but they certainly do the job. Distinctly lacking the subtlety of his public-facing persona, but isn't that the point? A knife in the kitchen is just a knife, sitting in plain sight, playing the exact role you expect it to. Outside of that situation? It doesn't matter to him anymore. By that point, you're the meat on the chopping block, and once the knife is rinsed clean and returned to the block, no one will know the difference.
Michael: He carries a flick knife at all times. Quick to retrieve, easily concealed but difficult to explain, legal to carry in minnesota, but a distinct red flag. Maybe it could be justified as a standard pocket knife, useful to keep on hand, but that isn't the point; They turn a disagreement into a knife fight, they're used to heighten the violence first and foremost, yet most people who carry them would say they're for self defence. For Michael? He couldn't quite tell you which. Not that he would want to.
Vanessa: Keeps a hunting knife in her car. Efficient, legal, utility first, used primarily for field dressing and making kills useful and presentable, but also to put shot animals out of their misery. The perfect pairing for a gun. If someone were to find it in her dashboard, sure, it might raise a few eyebrows, but it's easy enough to explain with a quick "My dad takes me hunting". Perfectly safe within plain sight. Made for cleaning up after a kill first and foremost, but it's plenty capable of doing the deed itself, if made to do so. It stays in its sheath until it's needed. Contained, kept sharp, and out of sight.