My DSIS and I fought continuously. Like your older DS I liked peace and my DSIS liked someone to play with.
Since we lived on a small estate with lots of DCs similar ages we never really escaped each other's company. I wanted to be left to play with the one girl my age, but DSIS and the other two girls who filled in the age gap between us always joined in.
It was a small town, there wasn't much extracurricular stuff, so we didn't have wildly different hobbies like my two do.
My DDs 16&13 don't fight much, never have.
Tried when they were 9&6, but I read the riot act and sent them to separate rooms every time they started and DD2 to her room when she started.
In their case it was generally DD2 making smart Alex, but I'll judged digs at her socially inept, dyslexic big sister that started trouble.
DD1 is very patient, but she much bigger than DD2 and just as clever. Both physically and in a real war of words DD2 would have lost.
Therefore, they weren't allowed to start.
Living in the middle of nowhere helps a lot, being friends is generally a win when there isn't anyone else to play with.
Lots of different interests a help too, they both do what they are good at and don't compete.
Screens help. When DD2 is bored she plays SIMs. Escaping into a virtual world for a while defuses many a tense moment.
Never underestimate the value of the wii, minecraft or the trampoline at getting the frustrated sibling to calm down.