Arcana Coelestia #1244
1244. That 'his brother's name was Joktan' means the external worship of that Church has been shown just above. That external worship is called 'a brother', see above at verse 21 of this chapter where Shem is referred to as 'Japheth's elder brother'. This explains why the word 'brother' is added here.
Genesis 10:20
20
These are the sons of Ham, after their families, after their languages, in their lands, in their nations.