Decoud's sister is hardly more than a literary device, someone to whom Decoud can address his long letter in Chapter 2-7. But I believe there are very faint clues to the effect that this sister betrays the gun-running operation to the Montero brothers, and helps bring about the Monterist uprising. Note that Decoud's second utterance to her concerns exactly such a betrayal. Pedro Montero's earlier presence in Paris is the kind of coincidence that could be deliberate on Conrad's part. The sister is described as "acute" and "intelligent"; although Decoud believes he can trust her absolutely, everywhere else in the book one character's knowledge of another is incomplete. She is described as being responsible for the well-being of her parents, and we do not know what their income is; in short, she has potential motive to sell Decoud out. Of course, she does not know that Decoud will go himself to Costaguana with the guns; when she learns, we may observe her (in careful parentheses) giving a stare of astonishment.

This exercise is purely for fun on my part; the novel reads perfectly well if one accepts the sister as a literary device.