That's a good question, because the relationship is so complex. On the simple biological/social level, the narrator is speaking in the first person. He's Luisa Santiaga's son, and has siblings mentioned in the story (brother of Margot, Luis, Jaime, and a nun). He is therefore involved first hand.
Add to that the following: Luisa always knew what was going on in the village, so we can assume the narrator got to tap some of that knowledge for what he shares with us about things. He is therefore informed beyond what a single person might be.
Now add the following: he's reconstructing events over 25 years later, so memory is probably fuzzy, and it is possible he's distorting things.
Now add one more factor: the narrator tells things he couldn't know, like what other people are thinking. This opens the question of who is really narrating the story, and if he really has the relationship to the community that the narrator is supposed to. If he does, how could he really know all this?