I’m not sure if your situation is a different beast altogether given they are sisters but when I rescued Ivy she was very possessive over food around other dogs. She was 6 months and had never had to really share anything or eat around any dogs before. With her what I did was have 1 person feed a dog behind a fence/gate and another person feed the other on the other side quite some distance apart. Then once comfortable with that over time (I mean at least a couple of days) then to progress to feed in the same room on opposite ends on a lead with 2 people, then closer and closer then eventually 1 person feeding treats to the 2 dogs almost next to each other. Treats is a different ball game to high value things like dinner out of their bowls or long chewable treats like bones. Do the same process but it may take a lot more time. If you see any growling or hackles rise or stiff stances do not proceed to the next step, you may even have to take a step back to try to get them used to it. If that doesn’t work then always separate the dogs with any food/high value treats. Also it’s good to always do training separately with the lister mates and then do training together too when they know what you’re looking for from them. Maybe charge a clicker and not use treats? I’m not sure what else I could recommend, definitely seek professional help and now, don’t wait for them to fight again because then it may become a habit and learned behaviour that may feel satisfy to them which you will most likely not to be able to stop or at least prevent you from living with them together. I’m sorry this happened to you, you seem like you love them both lots, I hope you all the best! Good luck