I obviously don’t know their background, but I know for my two those feelings come from having to compete for love, basic care, food etc so any difference between them sparks of a real fight or flight survival thing - it comes from jealousy, yes but it’s much deeper and more fundamental than “she got something I didn’t”.
I manage by being scrupulously fair where I can be - I don’t buy one something without having something for the other except for birthdays. At birthdays or celebrations I really involve them other child in planning and preparation- my DD can now be as excited about her brothers birthday as she is about her own. We celebrate every achievement equally, whether it’s star pupil, a good report card or a good gymnastics performance.
We talk a lot about fairness and are really transparent about why we did x for this person or y for that one, and we are ok with them expressing disappointment if we get it wrong. If one needs more of me than the other, we’ll talk about how we care for others when they are hurt or poorly. My DD would hurt herself because she knew if she was injured a grown up would attend to her - she doesn’t do that now but sometimes will be a bit poorly first thing in the morning because that’s less risky in her mind than asking for an extra cuddle.
In terms of food they each have a snack box and start the day with the same snacks, they can swap items eg if they prefer one type of crisps up another and once they’ve eaten their snacks for the day they’re done. Totally cuts out the “she got more than me” type squabbles - which come from a place where they both still fear there won’t be enough to go round.
It’s very hard but getting underneath the presenting behaviour to understand what’s driving the feelings is key. You might know you love each of them, that there’s enough food, or enough toys etc but they might not.