Haran was the first born son

Abram's birth determined by the time he left Haran

Ac 7:4 Then came he out of the land of the Chaldaeans, and dwelt in Charran: and from thence, when his father was dead, he removed him into this land, wherein ye now dwell.

Fact 1: Abram left Charan [Haran] when Terah died.

Ge 11:32 And the days of Terah were two hundred and five years: and Terah died in Haran.

Fact 2: Terah died when he was 205 years old.

Ge 12:4 ¶ So Abram departed, as the LORD had spoken unto him; and Lot went with him: and Abram [was] seventy and five years old when he departed out of Haran.

Fact 3: Abram was 75 years old when he left Haran.

205 - 75 = 130 : Therefor Terah was 130 when Abram was born.

Nahor or Haran born when Terah was 70

Since we know that Terah was 130 when Abram was born, and that Terah began having children when he was 70, then either Nahor or Haran was born when Terah was 70.

Ge 11:26 And Terah lived seventy years, and begat Abram, Nahor, and Haran.

Lot (Haran's son) is given deference by Abram

Some commentators mistakenly attribute generosity to Abram by the fact that he let Lot choose his portion of land first. The Leverite law stated that if the firstborn son died without an heir that the second son must bear a child on behalf of the first with the firstborns wife. The inheritance did not pass from the first to the second, but from the first to the firstborn's son. Abram's defence to Lot demonstrates that Lot was pre-eminent as the son of the firstborn (Haran).

Since Abram had brought Terah with him when they left Ur, Lot would naturally follow, being the heir apparent. Abram had the obligation to ensure that his dead brother's son received the inheritance.