My sister and I are one day less than a year apart. As teenagers, we fought over everything--the car we shared, clothes, everything. I can tell you that we are now best friends! It was when we left to go to college that we became much closer. We had our own space and really had nothing left to fight over. We live pretty far apart now, but talk on the phone all the time and have a really close friendship.
I know it drove my mother crazy too. Is there any way they could get more time and space to themselves? Like taking turns with one on one time with you? Also, maybe a relative or family friend could take them on outings together every once in a while. I know I felt less comfortable arguing with my sis when I was with someone other than my parents. Are there games you could play as a family where it would require them to work as a team?
They are also probably old enough that you could sit them down and have a serious talk. You may have already done this, but let them know how tired and sad it makes you feel. My mom once told me that sometimes she also wished she didn't have to come home to hear the fighting. It really had an impact on me and made me realize how much it hurt her. Tell them you need help--you don't know what to do any more, and that it's not right that everyone else in the household has to be around the arguing. All of you together can come up with a specific plan for whenever there starts to be a fight. Maybe it's going in separate rooms (neither of them in the shared bedroom) for an amount of time and cool off, then come together and talk about the problem/solution. Or, something else that everyone agrees is a good plan.
Best wishes!