why do you think dhal presents it in such a brief and emotionless manner?

The death of James's aunts reiterates a common theme of the novel - sudden death. Although James's aunts are certainly two unsavory figures (and may seem to deserve long, drawn-out punishment), Dahl deals with their deaths in an incredibly quick manner. This is similar to the death of James's parents, whose death was described in a mere sentence despite their positive relationship with James. All of these deaths reinforce the dark, random, even dreamlike nature of Dahl's often lighthearted children's novel.