Sorry, I'm not really sure. I just remember a few times Your Father was used when the context made it clear that they were referring to her son. Actually, I'm not sure if it ever properly used Your Son in dialogue.
Now that I'm looking at it, it seems like the names_bugged.txt from the patch link is missing Son from the text file. I guess it calls them up based on a word count, with 5 being Father instead of Son? Strangely the original names.txt that came with the game seems to also be calling her mom and stuff, so it already has incest?