OP,
Some siblings may clash, I have definitely heard of it.
In my case I have always had an absolutely zero tolerance for being unkind to each other.
It was always a case of "in this house we speak and treat each other kindly..end of".
Bla bla bla..
I stopped anything that they were doing and put them out in the garden to get "fresh air" to give them a "rest from their annoyance".....honestly I made myself the annoyance...not each other.
So if they were at each other, they would see me coming, after at a certain point, and they would say oh for flip sake she's here🙄😁.....
I had zero tolerance for any name calling of any sort..and it was never an issue.
They are really great friends, they treat each other very kindly, they have a great laugh together, which is really great to see...... with 6 of us at home at the moment.
I would advise you to bore the arse off them about being kind, pep talk them EVERY morning, first thing, ... be very firm about your expectations about how they treat each other.
We were all about ..."in this house we love each other and treat each other kindly and only use nice words" etc.bla bla bla...I was like a record.
I appreciate it may sound happy clappy, but they have never laid a hand on each other and get on so well, and are respectful....I'm so grateful for this.
I can honestly say it is definitely one of the things that I am most proud of.
They find me annoying at times🤷🏻♀️..but I am happy to take the hit if they get on with each other into adulthood and beyond my lifetime.
To have your children get on well, be close and loving, supportive of each other, IMO, must be the most wonderful legacy of a marriage.