"The Open Window" is a realistic fiction without much use of sardonic humor. How?
I have to start by disagreeing with part of your statement. I do think that there is plenty of sardonic humour in this excellent and funny tale by Saki, but at the same time, I agree that it is very realistic in the way that it is written and that Vera and Mr. Framton Nuttel are presented as real, believable characters.
However, the sardonic humour comes when we realise what an elaborate fiction Vera has spun for Mr. Nuttel, and how easily he has fallen prey to her skills of storytelling. What adds to this sense of sardonic humour is the way that Vera, without so much as batting an eyelid, then goes on to fool her own family in just the same way as she has tricked Mr. Nuttel by making up a story to explain his sudden disappearance when the "ghosts" arrive. As the last paragraph says:
Romance at short notice was her speciality.
This story then in an incredibly humorous way celebrates the power of storytelling and how we are all susceptible to a good story.