Factual error: During the battle scene the voiceover says that it is February 1915. The battle being portrayed is obviously the Second Battle of Ypres because it involves Canadian troops and it's the first time that gas was used in the war. The actual battle didn't take place until the end of April 1915.
Factual error: Prior to WW1, Colonel Ludlow picks up Samuel and Susannah at the train station in an all steel Model T Ford. This style of car did not come out until 1917. The correct car would have brass lights and radiator.
Factual error: The three Ludlow sons were leaving for England on October 14, 1914. When they were walking outside guiding their horses, the small trees in front of the house had green leaves. In Montana, Fall foliage typically begins by early - or mid-September. In most of the state, there should be peak Fall foliage (yellow/orange/red leaves) by early - to mid-October, not green leaves on the trees. Also, some time later when the sons were riding their horses through the field, the leaves were still green. (00:28:21 - 00:32:00)
Factual error: It has already been mentioned that, although the war scenes are set in early 1915, gas was not used until April of that year. There are further inaccuracies regarding this particular scene. The mask worn by the German cavalryman is not the sort of mask that was in use in 1915. It looks like a late WW1 or WW2 gas mask. Furthermore, since horses can also suffer from gas, a cavalry horse taking part in an attack in which gas was used would also be equipped with a gas mask. However, by early 1915, when both sides were dug into trenches, cavalry would not have been used anyway.
Answer: There's nothing to tell us who the listener is. Your theory that it's probably a family member seems entirely reasonable, but there's nothing in the film that would allow the identity of the listener to be narrowed down any further than that.
Tailkinker ★