Karna is the half brother of Arjuna, as he was born of Kunti before her marriage with King Pandu. Krpacarya's twin sister married Dronacarya. BG 1972 purports

Expressions researched:
"Karna is the half brother of Arjuna, as he was born of Kunti before her marriage with King Pandu. Krpacarya's twin sister married Dronacarya"

BG Chapters 1 - 6

Duryodhana mentions the exceptional heroes in the battle, all of whom are ever victorious. Vikarṇa is the brother of Duryodhana, Aśvatthāmā is the son of Droṇācārya, and Saumadatti, or Bhūriśravā, is the son of the King of the Bāhlīkas. Karṇa is the half brother of Arjuna, as he was born of Kuntī before her marriage with King Pāṇḍu. Kṛpācārya's twin sister married Droṇācārya.

BG 1.8, Translation and Purport:

There are personalities like you, Bhīṣma, Karṇa, Kṛpa, Aśvatthāmā, Vikarṇa and the son of Somadatta called Bhūriśravā, who are always victorious in battle.

Duryodhana mentions the exceptional heroes in the battle, all of whom are ever victorious. Vikarṇa is the brother of Duryodhana, Aśvatthāmā is the son of Droṇācārya, and Saumadatti, or Bhūriśravā, is the son of the King of the Bāhlīkas. Karṇa is the half brother of Arjuna, as he was born of Kuntī before her marriage with King Pāṇḍu. Kṛpācārya's twin sister married Droṇācārya.