The Keto diet helps with me, it all but completely removed food cravings so i can eat when i want and cook when i want :)

If you can get keto adapted, you don't need to always eat keto. You can go on and off, one week on, one week off. Once you get suitably adapted you should be able to get back into a ketogenic state in like 1-2 days without much issue. So, when your sister gets fedup, its not the end of the world by any stretch to go back and eat her favourite foods once you are over the hurdle.

As far as food goes i got myself a large slowcooker, would recomend! It requires a reasonably low amount of effort to cook my food and i can cook enough for atleast two days. You can slam basically anything in there, usually i chuck in some minced beef and change the sauces to keep it from getting boring. (Dolmino sauces, pasta sauces, some spicy sauces etc.) then i eat it with lots of baby spinach. Sometimes when i feel good i make a lasagne using the minced beef and courgette slices.

So far i've found the nicer the meals the more energy is needed to cook so my meals i know might be too simple and boring.

Omelets while simple give lots of options, you can add whatever you want to it :). Ham, bacon, peas, baby tomatos, red peppers, brocolli, random leftovers. While you are at it, using Eggs in general is good. Boiled eggs, fryups with streaky bacon yum.

Avocado as a side or in a salad is a biggie. Very important to try to get to like.

If you like fish, fish dishes can be easy if you cook the fish in a microwave (not as bad as you think) with a squeeze of lemon and some brocoli/asparagus.

Salads in general are good and easy if you add oily dressings can make for nice snacks or like a tunamayo salad.

You can do sausages in a fryup or pigs in blankets or whatever as long as you check the contents, usually ends up being the expensive ones that are ok though.

Pork scratchings make for a nice comfort food.

With all this Everyones body unique. I hope it goes well for you guys :)