I'm in the middle of this right now

I have been feeding filling and nutritious "snacks" between meals so they aren't very hungry for the processed junk: beef jerky*, cheese (our raw one when I can

), nuts, natural peanut butter (on spoon or in our pita bread, maybe with a little stevia and/or fresh fruit), Lara bars for hikes, sprouted cinnamon/raisin bread with lots of butter, fresh fruit, eggs...
*I made beef jerky before we came with ground beef and beef liver (processed to be smooth, seasoned, and then made into thin strips...I used a ziploc with the corner clipped to pipe long strips onto parchment and then covered them with another sheet of parchment and smashed them as uniformly flat as I could...we dried it ala Alton Brown with a box fan and cotton a/c filters).
I also make extra veggies for the meals since she does a lot of canned stuff and starchy veggies. I just lightly steam some fresh veggies and make sure they're ready for the kids to snack on while she/we are finishing making the main meal.
I have been eating lots of CO "cubes" I brought with me...flavored with chocolate, shredded coconut, and peanut butter. These have really helped ME to avoid the cakes, caramel/chocolate bars, white breads, etc.
(Thankfully, ds1 didn't like the icing on the cake when he tasted it on the beater so I had a good excuse to sautee a cubed, slightly-overripe nectarine with butter and a little stevia and serve warm with some of the not-too-terrible vanilla ice cream everyone was eating

...it was much better in my opinion!!)
We brought a little bit of food with us, but it's mostly gone and since we're in the middle of nowhere the grocery store has almost nothing to offer. We miss our kefir and yogurt and kombucha!

She only does conventional skim milk, so I'm breathing deeply

and I also brought a couple of cans of coconut milk to supplement.
Also brought the CLO.
I scrub the fruit as best I can (hard with strawberries

).
I move the yucky HFCS pancake syrup next to the pan so they can put i ton their's but it's off the table so my boys don't ask (hello, it's yucky and then she microwaves the plastic every time they use it!).
I water down their juice (which we don't usually drink).
Dss are still young enough that I fill their plates so I have some control there, but they are old enough to ask for things others have on their plates so I breathe deep and serve small portions when I feel like they'll care.
My MIL really does try to help us, she just doesn't "get" it

. She does keep butter around now which is very helpful (I guess she got the hint after I'd bring it with us

)
Good luck!!!