Not sure if youre looking for advice our just to share with other posters in the same boat? Mine are 13, 11, 9 and 6 and so far they are well behaved. Whether this is luck or something we are doing, I'm not sure.
But fwiw
I think they need some autonomy as they get older, so make sure they're not over scheduled and have plenty of time just to 'be'. I let mine self regulate, so I don't chase them to do homework or tidy up at a particular time, they just know we expect it to be done. I don't argue about minor stuff like wearing a jacket when its raining, but I will insist on them taking a phone if they're out on their own. No one likes to be told what to do all the time, it's a wind up.
I did let them go into town on their own at those ages, they went to the library, sweet shop, charity shops on their own. After meals they clear up and get the dishwasher on. They keep their rooms in a reasonable state, produce their washing and put it away. They also do a lot for the dog, and help each other. I think if they take part in the running of the house they feel useful, which helps their independence.
As far as getting on with each other, if one is upsetting another we encourage them to stand up for themselves, I don't step in to tell one off, just get them to learn how to make it clear that they've had enough of the other's behaviour and ignore or walk away if necessary. We did read the 'how to talk books' especially 'sibling rivalry' years ago, which are excellent. I also don't tolerate whining and complaining, they have to restate what they want as a sensible request. So 'I can't open this' becomes 'can you help me open this?' silly example but I'm not a complaints centre...'I'm bored' gets 'would you like a list of jobs to do for me to keep you entertained'.
I also think eating together at least once a day is a massive help in terms of bonding as a family.
I know I will be accused of smuggery for this post. In real life when people complain about their dcs I don't ever, ever say , 'Well mine are lovely' or 'here's what I think you should do...' as it is social suicide. I usually just listen and sympathise. But maybe someone reading this is looking for solutions, and some of these might work for someone.