Wow, I feel your pain. Our daughters are closer in age than yours and I know the problem.
Here are some thoughts. One, you need to sit the girls down and have a talk with them. Do they understand that you need to spend time at the business right now, and that it's for the family's well-being? They may be feeling resentful that you are not available now, and if possible you should try to spend time with them when you can. Still, they are old enough that you should be able to enlist their help as allies in the process of supporting the household. Their job, among others, is to get along together so you can do the work you need. Perhaps they have other jobs too, like certain chores they have to do each day. If not, it would be good to have some structure like that. Most importantly, they need to learn how to work out their issues without running to you each time they fight. Our preschool teachers instill that among the 4 year old classes, that the kids have to talk with each other and try to work things out first.
I definitely understand the challenges of being in a small place, but try to find a space where each of them can go to have their own time/ space. Set up a separate hangout space for each one so the girls aren't on top of each other all the time.
Try to arrange for each of them to have things to do this summer -- are they working? if they can't find a job, have them volunteer somewhere. Help at a children's camp, or find another opportunity for each of them to have her own activities separate from the other sister. There are tons of volunteer opportunities available, and you can simply require that they choose one to participate in.
If they call or text you about a fight, you need to try to not get involved, not resolve it for them, but be firm about encouraging them to work it out.
If they need to be home alone a lot, and the fighting/ contacting you continues, then you may need to literally bring one or both of them with you to work. They will have to sit quietly and read a book, or help you if that's possible, or whatever. I would give them the choice: either get along and work things out between themselves, or they can come to work with you. And then if need be, follow up and do that if they can not figure out how to manage otherwise. I have a feeling they probably wouldn't like that very much and that might just prompt them to get along better.
Good luck!!
Finally, I would try to read some parenting resources like the Love & Logic materials and Adele Faber's Siblings Without Rivalry (and her other books too). Those will give you some good ideas about how to make changes that might help.