The speaker uses formal language to tell us of this crime. It's a curiously detached way to speak of it. "My father" is a distant way to speak of that relationship. Calling him by his full name is also unusually precise. That name, "James Witherspoon," has an English flavor, which we often read as more stiff, and also sets this in a legal context where bigamy is a crime. Then "bigamist" is the legal term, also cool and indirect.
The two commas create pauses that add to the impression of speaking rationally and reflectively. The narrator's father has committed a crime – a shocking crime within that culture. But the narrator also speaks of this as distantly as possible.
From this one sentence, we expect the story to unfold the consequences of this crime through close observation and muted feelings.
How much differently would you expect the story to go if the first line was, "My pa Jimmy has two wives?" Would you be more or less intrigued if there were more emotionally laden words added to the sentence, such as, "My dad, James, is a lying, two-timing, worthless oath-breaker?"
As it is, the two characters in the sentence – the narrator and the father – stand apart from each other, and the narrator looks at the father more to think about the situation than to feel it, at least on the surface. The crime in this sentence creates tension. The tone of the sentence suggests how the story will approach that problem.
The more a reader likes that voice, the more drawn they will be to continuing the story. The sentence promises to think about a difficult situation.
Graphic design by Ken Silbert