The younger brother in the movie is indeed very lucky to visit his brother's house at the right moment when his brother is on a business trip, leaving only the sister-in-law at home. It's true that a fire near straw will eventually burn; the time without her husband next to her has made Suzuki unable to control her desires, sudden urges just like any other married woman. This makes her feel very restless and uncomfortable, and she has to find a way to free herself. Of course, in the small house with just the two of them, provocative actions towards her brother-in-law to help her achieve that desire are entirely understandable.