As narratives go, that provided by the narrator of “My Side of the Matter” is about as subjective as it gets. The events recounted in the tale are provided to us courtesy of the narrator's one-sided perspective. His skewed version of events is the only one that's available, forcing us to decide whether or not we should believe what he's telling us.
The title of the story gives the game away; this is the narrator's side of the matter. But there are two sides to every story, and because we never get the other side, at the very least we must remain on our guard, healthily suspicious of what the narrator is telling us.
One must also bear in mind that the narrator of the story is only sixteen years old, an age at which young people's imaginations are still working overtime. It could well be the case that his wife's maiden aunts really are trying to kill him, but given his age, it's much more likely that the narrator's just spinning a yarn.
Either way, the narrator's lurid and unflattering descriptions of the maiden aunts, though undoubtedly amusing, are somewhat exaggerated, indicating that perhaps the narrator is more concerned with telling a good story than with giving us the unvarnished truth.