Albert Barnes Commentary Judges 8:19

"And he said, They were my brethren, the sons of my mother: as Jehovah liveth, if ye had saved them alive, I would not slay you." — Judges 8:19 (ASV)

The sons of my mother - A much closer relation than that of brothers by the father only. (Deuteronomy 13:6; Psalms 69:8.) This is the only hint preserved of the transaction. We cannot say exactly when the slaughter of Gideon’s brothers on Mount Tabor took place, whether before the outbreak of the war (Judges 6:33), or in the retreat and flight of the Midianites (Judges 7:22).