she doesn't need to "learn" to eat. she knows how to eat. she does it just fine.
this has become a control issue and her mom probably started it, and dad is continuing it at your house, by shouting at her and fighting with her about it.
my niece (who will be 6 in a couple of weeks) is very similar. i imagine if her parents were divorced and she was in your step daughter's position of "visiting" one of her parents, and all the emotions that go along with that, she'd probably be JUST like your step daughter.
when she is with me, she eats very little most of the time. she's not with us often (similar to your step daughter) so when she is coming, i make sure the only snacks or meals i plan, are healthy. then, if she eats, great, if not, that's her choice.
funny story. the first time she tried to pull her, "*SIGH*...i'm just not hungry" with these huge eyes and all this drama...i just said, "okay". she looked at me like i sprouted two heads, for not yelling at her and starting a fight.
SHE controls what she eats. we have zero control over her putting that food in her mouth. what we can do is model healthy eating habits. and no, if you don't eat a healthy meal, you don't get the junk food for dessert or snack later. but i seriously could not care less about fighting with her over eating, the small amounts of time i get to spend with her.
your hubby has to stop hounding her about it. give her healthy food options ONLY. and then back off....and see what happens. tell her from now on, she decides if she eats and how much she eats. the only thing YOU and her DAD decide, is what is offered. completely put the power back in her hands (it's there anyway, which is why she keeps playing the game). accept this is a battle you can't win. you AND dad. you both have to do it. i bet you will see things change if you change the focus of your time together.
(i am answering here because your other post has no question and is really just whining about answers you didn't like. hon we all have to put our big girl panties on sometimes, and admit we don't know what we're doing. that's why you posted the question, right? hang in there. it'll get better. you can't change her, you can only set reasonable rules and expectations. thinking you can control what she puts in her mouth is not reasonable.)