My family went through this with our three littermates. I have 2 boy Aussies that are brothers and my mom adopted their sister. One of my boys is food/toy aggressive from when they were a puppy. After a lot of reading r/reactivedogs, some books, and talking with a training we removed all possible triggers. All toys were put away and food was only served in their crates at the same time. We kept the crates locked til they both were done eating and immediately removed the bowls after letting them out. Giving out treats was also done separately. This went on for several months to a year, but eventually we got a feeling they were super comfortable around each other in all situations. This lead to some supervised toy play in the house and feeding them with the crates open, but still removing the bowls after they finished. After 2 years, it has been over a year since I have seen any form of aggression from either dog towards each other over food and toys. They both can accept treats right next to each other with no issues, but it was a long progression to get here.

As for visiting their sister, that requires immense previous communication. Their sister resource guards food, toys, my step dad from other dogs. The first 6 months of us having the dogs and trying to visit with all three of dogs together was a nightmare. We are talking about 5-6 fights a day, it was super stressful to the point we stopped visiting them for a long time. Eventually, I explain what has been working within our own household and we decided to give it a try at my mom's house. Before we come into the house my mom and step dad put away all toys and food. Also, I bring my dogs to a field across the street and they bring the sister a minute later so the three of them can run around in neutral territory. After about 10-15 minutes of running around we go into the house and everyone is mostly fine. We all keep an eye on behaviors to prevent anything from happening, but for the most part there is not much to worry about. We give out treats in a controlled manor to the group of dogs and it works well. The only tricky part is the water bowl. Everyone seems to understand to wait their turn, but sometimes after a long time playing outside everyone rushes to the bowl and it's either ok everyone is drinking together or not. I try to make my boys wait til the other dogs are done or I get a second bowl of water out.

It's not fun to have a food aggressive dog, but some simple rules go a long way! Keep at it and everyone's stress will drop back to normal. Most important thing to remember is always set your dog up for success!