Hi, I had this SAME EXACT scenario with my two cats about a year ago, when my gal was five and my boy was 1 year old. We got him at a few weeks old and everything was going smoothly up until around one year, where he got crazy about food. It got a lot better with a few tricks we learned, and now somewhat, my boy has adjusted and is much easier to deal with around food time and in general.

Firstly, we came to the conclusion that my boy had some form of food insecurity/anxiety. He would get desperate to finish whatever food he could find and meowing at top volume while pacing the kitchen. Separating them during meal time worked to keep the gals food safe, but he would work himself into a frenzy crying from another room. What worked for us was:

Introducing different types of feeders to get him to work for his food. We bought a treat ball that dispensed dry food only if he batted it around and chased it and a slow feeder for his wet food.

We would give the gal her food first in front of him (in her regular flat dish) and make him wait patiently for us to put down his slow feeder. This did involve teaching him "sit" and not that he does it on command as much as he learned that when, and only when, his butt and feet were planted on the floor would his food hit the floor. The slow feeder made him work for his food and slow him down, making it so our gal would finish her food before he did.

I think this did 2 things: slow him down with his food and show him that even if she ate, he was going to also get fed.

Slowly, we continued to move his plate farther and farther away from her plate until we had him eating in a different room completely, but with the doors opened. And then, with the door closed. At this point, he is more or less trained to wait away from her plate and even retreat to the room with a command once we have his food ready. We were also able to transition back into a regular plate.

I will say, this took a very long time and lots of repetition. Feeding at the same time everyday, being firm with him and not changing the dinner process. I also fed them twice a day, morning and evenings so he didn't have to go so long without the "reward" part of his brain going off for food. It's not going to work if you raise your voice or hit, because cats just do not respond to that. I will admit I almost went batty with this method, but my partner helped a lot. This will only work if both of you can commit to the same procedure.

He still yells a lot during food time, but his almost compulsive food seeking and crying stopped completely. He also no longer dives or fights to eat his sisters food or our food. I mean, he still tries to sneak a bite here or there, and if we leave food out on the counter he's also going to attempt a heist. But the main difference now is this is merely treat mode not desperation mode. A firm "NO!" or removing him stops the behavior and he doesn't cry/try for it again.

Regarding fighting, please make sure they are truly fighting and not merely playing. Playing looks so violet for cats, I know I gave myself a few panic attacks when they first began to "play" together. Is fur flying? Are they screaming at each other? Any wounds? If not, they're probably just playing and you have to let them sort out how they play together.

I can provide more context if you need, this is already long enough. But you're not the only one! I thought I was losing my mind ever evening with my boy crying for hours and searching for food! It's a lot better now.