Question on Terrence Malick's "Days of Heaven"(1978)

I'm struggling to understand the point of two of the main characters posing as brother and sister in the story. It seemed like the reasoning was they didn't want the other workers to gossip, which they obviously were anyway when the brother and sister are getting frisky in the open fields together.

As a plot construct this moves the plot along, and while they could have opted for a straight love triangle in the story, the choice just feels incredibly odd.

Any thoughts on this?