Hmm, Julian is the only one who thinks she's dead, Charles seems incredulous when he says so, and other people address her directly. Constance maybe wouldn't be so concerned about her handling knives, matches and food if she were dead.
Does Merricat directly interact with Julian? She is always telling Constance to do things for him, making suggestions, but she never directly does anything for him herself. Does he ever talk to her? He says "my niece" and always means Constance. She must sit in the corner, is not allowed to prepare food, and is not allowed in Julian's room or to touch anything of his things. She also never sits at the table, her food is brought into the kitchen after they eat.
Julian was nearly killed by Merricat, but doesn't know that. He is constantly trying to relive the day and piece together what happened, but he also asks Constance (constantly) whether it really happened at all. He is an unreliable witness - he was never called to the stand at the trial of Constance, though he was the only person who was there left alive.
I do think the ambiguity is intended; it is deliberately disorienting, but I don't think Merricat is Bruce Willis.