I'm speaking as a child who fought with younger sibling a lot to a point we hardly spoke for years. He's quite a decent person now 😁
For a start off, how do you respond?
"You're making me so stressed" "I find it so upsetting"...
Thing was, I always (or almost always) felt I had a good point, and he was deliberately being annoying and why should I let it go? So when she responded like that, I felt guilty for upsetting her, but really it was his fault and if he wasn't so annoying then I wouldn't have had to react. So that made me more annoyed with him.
Do you honestly treat them both the same when they fight? Or do you blame the older one?
My younger sibling was acknowledged to be "difficult". I think honestly dm didn't really know how to deal with him, so it was easier for her to not address when he was clearly the awkward one I was always the one who had to compromise - and I did 90+% of the time.
Don't make excuses for their behaviour to each other.
Frequently if he did something nasty to me, if I complained then I got "He's having a difficult time" or "he doesn't understand" or "he is just upset about <irrelevant incident>". Fine, maybe, but if he'd just broken something of mine, it was still as broken, and I was the one that lost from it.
If they say something then listen. It doesn't matter if you think it's a really silly thing that upset them. It may mean a lot.
There was a particular thing that really hurt me as a child. It wasn't really younger sibling's fault, but he gained from it. I approached dm as an adult to try and explain how upset I had been. It very sillily still does upset me today if I think about it.
Her response was first to tell me that I hadn't minded at the time - I remember desperately battling against tears, and trying to hide it. Then when I said it had, she said something along the lines of "well, I don't know what could have been done." All I wanted was an acknowledgement, or an apology. As an adult I'll still say it was very unfair, and there was a very simple solution too, and tbh if they'd been trying to be fair and thought about it, it would never have happened.
Don't praise their siblings up to each other to try and make them see the other's good points.
"He's such a sweet boy." "He loves you so much." "He looks up to you." In which case, why wasn't he nicer? I saw no evidence of any of the above at all. All it made me do is feel unloved and that they thought I was the only issue.
Don't make them do everything the same.
Anything I did. My younger sibling had to do too. And I couldn't win. If I was better - naturally because I was older (and please don't say too much about it in case younger sibling is upset). If he was nearly as good, this was amazing and had to be shouted from the houses.
I remember hearing dm talk about the musical instrument we both played. "Yes Margaret plays. Little bro also plays and he's got the most wonderful technique, he's got so into it and his teacher says he's going so well..." Actually at the same point, I had done grade 5 and he'd failed grade 1 twice. So I think dm thought he needed confidence boosting, what actually happened was I gave up the instrument, which I think I could have been really good at, because I felt there was no point.
Don't compare.
Obvious isn't it? But you will compare. Don't say it out loud, and watch how you say things that can be seen as comparing.
I felt constantly compared to both my siblings and found wanting. I thought I wasn't academic for example. In my 20s I suddenly realised that I had better GCSEs, equal A-levels and what most people would probably count as a better degree.
Never realised as a child. So I didn't bother working. You see it was easier on my mind to not work, then when I didn't achieve, to tell myself that if I'd worked, then I would have done as well as them. Just didn't notice that even without working I was achieving as well as them - or better.
It wasn't that they said much out loud, but I always felt that there was an air of "poor little Margaret tries so hard, but she'll never be up to them...."
If one of them is doing deliberately annoying things give them a consequence.
Dm used to say "he can't help it" or "I'm sure he didn't mean it." Actually when it had caused issues for me, then even if he didn't mean it (and he'll admit now he did) then seeing him "get away" with it was frustrating.
Don't make them lend/give stuff away.
Lost count of the number of stuff I was told I "had" to lend, and it came back broken. Never got it replaced either.
Try and look fair.
There's always times when we can't be fair. But be prepared to justify when they don't. If you can't justify it, then probably it's because it isn't fair.
For example, if I bought myself something or got something for birthday/Christmas that I really liked, and my younger sibling wanted one. Dm would buy one for "herself" and then "let" him use it. She'd tell me he was only "using" it and it wasn't his, but it remained the fact that basically he got one for free.
And if you ask the sibling for opinion if they think something's fair, then listen to them.
I tended to take the line of least resistance, so if I was asked, I would almost always say it was fine. I suspect because of that, if I did say it wasn't fair, dm wasn't prepared so would then tell me she'd thought carefully and was still doing it. I felt my opinion didn't matter.
Praise
Praise them and don't hide it in front of the other if they have done something well. I remember dm coming to me after a particularly difficult holiday where I'd given up on the family tradition of children do the washing up after meals, and just done it by myself every meal. She came to me and whispered thanks, gave me some chocolate and told me not to tell younger sibling.
Do these things
Find things they can enjoy together without being compared. And praise them, not for getting on with each other, but each for doing it well. So not "thank you for getting on so well together" but "thank you for tidying the room, you both put a lot of effort in".
Have time with them separately. If the other one tends to interrupt, arrange to go out.
Let them have separate hobbies and friends, and keep siblings out of the way when friends are round.
Make them laugh. Laughing together really helps in relationships. Watch something on TV that they both find funny, play a game that they enjoy.
Be fair. eg If older got his first phone at 11yo then younger does too.
Be prepared to give consequences. And give equal consequences, not less to the younger one.
And just to note, I love my parents. I totally understand that they are human, they were doing their best, and there were things that at the time I didn't understand. But I think they did contribute to our bad relationship, mostly through being so desperate to mend it, they tried to push us together, which actually drove us apart.