Having discussed Ludendorff and Poison, lets take a moment to delve into what we know of
Steve Trevor.
Not much, on the surface, as you might expect from a spy. But there are a few clues.
We know he had a father who helped raise him, who was likely also in the military and raised some hell. He seems to know how to ride a horse pretty well, so we might surmise he had a country upbringing instead of in the city. Considering he carries his father's watch with him we can assume they were pretty close
A little more is revealed in his conversation on the boat with Diana where he talks about couples loving each other til death, but then tells he it rarely happens. A comment like that could indicate his father and mother [who he never speaks of at all] separated at some point, and that Steve was then raised by his dad. Further evidence for this comes from his scene dancing with Diana in the snow. He says people not at war "get married, have babies and grow old together" but also that he has no idea what that is like. Again, that hints that his own parents did not have a happily ever after for him to draw on.
For all his charm around Maru at the Gala, he never comes across as particularly suave with Diana. Quite the opposite in fact, as demonstrated by his old fashioned chivalry and not a little nervousness on the boat about sleeping next to her. He even covers up [eventually] when she is with him in the bathing cave. While he is quite comfortable in his disguise with Maru, he is often flustered around Diana, and has difficulty opening up to her. Again, at the risk of getting overly Freudian, it might show he did not have a close relationship with his mother or that he felt abandoned by her.
Finally, I think we can assume by the time of the movie Steve's father has passed away. He gives his dad's watch to Diana, but never asks her to return it to him, or even to get a final message to his father. Considering his dad obviously had a pretty strong influence on him, I would think that were he still alive Steve would at least have wanted her to get word to him of his son's death. So it seems that Steve, like Diana, is alone in the world... until they all too briefly find each other.