You really should've told your brother Sumie... That's the difference between humans and parasites, emotion comes before logic for most of us. I think she knew that he was "something", but didn't fully believed herself and went to talk to him hoping she would be wrong, but guess what? You weren't... At least she's armed with some unknown chemical. Hope it helps.
I gotta say, I'm loving every single panel of this manga. Iwaaki's commentary on the entire human race as a species and the human emotion/soul X reason/logic is incredibly well portrayed in the pages. Not to mention the way he wrote a foreign species learning, understanding and adapting to human society.