I'd like to ask about the meaning of the phrase "Don't for two moments think otherwise" in the following context
One old aunt assured him, "you will go insane in the darkness. Starlight has that effect on people, you know."
That wasn't true. Humans were adaptable, and besides, the New Isles were lit up with blue-white lights very much like sunshine. Yet his response was deflected with a cold pleasure. "You will go insane," his aunt repeated. "Don't for two moments think otherwise, my boy."
The situation in the passsage is as follows: an aunt is talking to his nephew who is going to leave his home in connection with a potential danger of poisonous and flammable gas eruptions.
My guess is that "Don't for two moments think otherwise" means "think carefully, think twice before doing that."