KJV: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
NIV: But I tell you that anyone who is angry with a brother or sister will be subject to judgment. Again, anyone who says to a brother or sister, ‘Raca,’ is answerable to the court. And anyone who says, ‘You fool!’ will be in danger of the fire of hell.
ESV: But I say to you that everyone who is angry with his brother will be liable to judgment; whoever insults his brother will be liable to the council; and whoever says, ‘You fool!’ will be liable to the hell of fire.
CSB: But I tell you, everyone who is angry with his brother or sister will be subject to judgment. Whoever insults his brother or sister, will be subject to the court. Whoever says, ‘You fool!’ will be subject to hellfire.
The phrase was probably added to make explicit what Jesus probably meant. There are actions a brother or sister can take that could result in justifiable anger.