Here is a list of some brothers who killed their brother/s, although I’m sure I’ve missed some:
Men of Levi (not necessarily literal brothers)
Exo 32:26-29 then Moses stood in the gate of the camp and said, “Who is on the LORD’s side? Come to me.” And all the sons of Levi gathered around him. (27) And he said to them, “Thus says the LORD God of Israel, ‘Put your sword on your side each of you, and go to and fro from gate to gate throughout the camp, and each of you kill his brother and his companion and his neighbor.'” (28) And the sons of Levi did according to the word of Moses. And that day about three thousand men of the people fell. (29) And Moses said, “Today you have been ordained for the service of the LORD, each one at the cost of his son and of his brother, so that he might bestow a blessing upon you this day.”
Abimelech – 69 brothers
Jdg 9:5 And he went to his father’s house at Ophrah and killed his brothers the sons of Jerubbaal, seventy men, on one stone. But Jotham the youngest son of Jerubbaal was left, for he hid himself.
Absalom – Amnon
2Sa 13:28-29 Then Absalom commanded his servants, “Mark when Amnon’s heart is merry with wine, and when I say to you, ‘Strike Amnon,’ then kill him. Do not fear; have I not commanded you? Be courageous and be valiant.” (29) So the servants of Absalom did to Amnon as Absalom had commanded. Then all the king’s sons arose, and each mounted his mule and fled.
Solomon – Adonijah
1Ki 2:23-25 Then King Solomon swore by the LORD, saying, “God do so to me and more also if this word does not cost Adonijah his life! (24) Now therefore as the LORD lives, who has established me and placed me on the throne of David my father, and who has made me a house, as he promised, Adonijah shall be put to death today.” (25) So King Solomon sent Benaiah the son of Jehoiada, and he struck him down, and he died.
Jesus prophesied that brother would deliver up brother to death:
Mat 10:21 Brother will deliver brother over to death, and the father his child, and children will rise against parents and have them put to death,