I grew up with a brother. I know it is natural for siblings to bicker and fight sometimes. I also know it is easy to put pressure on their individual relationship when you are addressing the behaviors. I hear parents saying things like, “you will love your brother. He is going to be your best friend some day.” and, “In this house we will all like each other!” This pressure tends to weigh heavy and if anything, backfire.
The idea is to address the behavior and teach the social skills in general and let that trickle down to the sibling relationship. If they are name-calling, teach and practice how we speak to people, make a rule that we all call each other by name. If they are grabbing toys, teach about turn-taking and sharing without mentioning siblings by name. Whatever the difficulty, go broad and focus on skill building first rather than directing them back to each other specifically.
During all of this, focus on giving them opportunities to share play and space. Provide fun projects and outings together. You are giving them a good chance to practice all the social skills you are teaching without forcing the flow.