Ah OP almost identical circumstances and issues with my step child. Mum provides him with a horrendous diet, McDonald's, pot noodles, kebabs, Domino's nothing remotely balanced. Lives with us half the week so we end up trying to encourage healthy choices. Things that have worked for us-
Hidden veg dishes in things like chilli, pastas etc. I tend to mix the chilli in with the rice so that the bit he would go to first (the white rice) is already mixed together so he's less likely to eat around the other stuff.
Roast dinners, step son also loves his potatoes, Yorkshire pudding and gravy. So when it's plated I put the bare minimum potatoes and YPs on everyone's plates, when he asks straight away for more of either we tell him yes there's more once you have eaten your veg I'll go and get you more if you want.
Fruit, he's so picky with eating fruit so I always make sure that we have things he likes (melon, red grapes) in, he also seems to really like things that are specifically for him so I bought pink lady apples and showed him the little pink fleshy bit under the skin and said the four there were for him and he enjoyed that.
Plates, I give him less than I would give his younger sibling, big heaps of food he gets overwhelmed, I use a slightly smaller white plate and space things out so he can see what he has.
We've stopped making a big battle of it, well I have, I try to be more jokey about it with him and we "trick his dad" in surprising him by eating the good stuff. He and my husband lock horns a bit about it still and we don't get very far, like yesterday became a bit of a "well I'll sit at the table all day if I have to I am not eating any vegetables" then my husband left the room for something and I asked if anyone wanted the last piece of fish, step son straight away said he did so I suggested he ate some veg first, he'd finished the veg before my husband came back in.
These are just things that work with my step child, he isn't remotely overweight so that factors in to him refusing healthier food and asking for big quantities of highly processed foods. He's also quite immature in some areas so despite being the same age as your step daughter things like the above work.
Also, I wouldn't have given any of my children cake and ice cream if they'd left the bulk of a meal either. We don't need clear plates here but you also have limits as a parent in terms of empowering your child to eat until they're full and not overeat and then them just being children and wanting the sugary stuff.