As someone who grew up with two brothers and also a mother to 4 children who are much older now, I have found that when it comes to siblings having their differences, it is always best to leave them to try and resolve the issues themselves, but always keep an eye on them, should you need to intervene.
I feel if you intervene too soon, you miss the opportunity to see whether your children can resolve the issue themselves, which is huge when it comes to development and social skills.