What is the situational irony in "The Open Window" by Saki?

Let us remind ourselves of the definition of situational
irony. Situational irony is a specific form of irony that describes an occurrrence that
is the opposite of what we expected. An example would be a rags-to-riches story, where
the poor beggar suddenly inherits a fabulous fortune, or its reverse. Clearly, when
thinking about this excellent story by Saki, there are a number of different kinds of
irony. The situational irony though that is most important (especially to Mr. Framton
Nuttel), comes when, after Vera has told Framton about the terrible tragedy of her uncle
and cousins, and we, like Framton, are led to believe that her aunt is somewhat deranged
with grief, these supposedly deceased members of the family walk straight in through the
open window just as they would have if they were still alive. Note the response to this
event:

Framton
grabbed wildly at his stick and hat; the hall door, the gravel drive, and the front gate
were dimly noted stages in his headlong retreat. A cyclist coming along the road had to
run into the hedge to avoid imminent
collision.

Clearly, the
situational irony of the story is a great shock for Framton Nuttel, and given his
delicate nervous condition, we are left thinking that Vera is rather cruel for the
amusement she takes in tricking him.