Which passages in the story obviously have humorous intent?
I think that Gordimer's story is constructed with so much in way of taut focus that there is little left over in humor. The manner in which Gordimer describes the mother- in- law is where some humor can be found. The husband's mother, described as "a wise old witch," is where some humor can be found. This is seen each time the husband's mother is referenced.
However, outside of this, there is not much in way of passages that use humor as a direct element in the story. Gordimer's primary purpose is to explore the fundamental fear that the family experiences. This "fear of the other" is incrementally paralyzing, and is taken so seriously, that there is little in way of humor evident. It is because of this where I think that Gordimer does not feature much in way of directly obvious humor references. It becomes evident that Gordimer seeks to make a larger statement about where White South Africans are in the relationships they have with their world. This statement is one of pensive reflection, carrying with it serious implications, crowding out any potential room for direct humor.