Mahabharat

Pandavas

Pandavas were sons of Dhritrastra's brother Pandu.
Pandu had two wives, Kunti and Madri.

Pandu accidentally shot Sage Kindam and was cursed to die if he tried to touch a lady.

Since Kunti had a mantra to call upon the Gods for children, Pandu asked her to call upon Yam, Pavan, and Indra.
Kunti shared that mantra with Madri and she had two sons from Gandharvas/two Aswini kumaras.

Yudhistr was son of Yamraj (God of Death and justice) and Kunti.
Bhim was son of Pawan Dev (God of wind) and Kunti.
Arjuna was son of Indra Dev (God of Rains and king of Devatas) and Kunti.
Nakul was son of Madri.
Sahadev was son of Madri.

Before marriage Kunti tried to test the mantra on Sun God and had Karna.
He was abandoned by his unmarried mother and was destined to fight against his brothers.

Pandu had died when Pandavas were kids, and Karna wasn't aware that he was Kauntey when he chose sides, even before there was any sign of war, and had taken Arjuna as his competitor.

Hence Yudhistir was eldest on Pandavas side, and had Sri Krishna as a guide and strategist.