Maybe we can look at his asking her to eat as his way of being concerned about her and wanting her to eat something. Which you would want too. So that is a good sign.
Hopefully the doctors can figure out soon why her appetite is low, and help it. I know from experience that when I'm not hungry, it's hard to "force" myself to eat. If she's just not hungry, she may be in that situation.
In the meantime, there must be a way to separate your own emotional health from what they do. Your happiness needs to be separate from what they do or don't do. Maybe consider them a scientific experiment of aliens that you can watch without being bothered by it?