2 Chronicles 21:3
And their father gave them great gifts of silver, and of gold, and of precious things, with fenced cities in Judah: but the kingdom gave he to Jehoram; because he was the firstborn. And their father gave them great gifts of silver, and of gold, and of precious things, with fenced cities in Judah: but the kingdom gave he to Jehoram; because he was the firstborn.
The father in the verse is Jehoshaphat, who was succeeded in the kingdom by his firstborn son Jehoram. The other sons of Jehoshaphat named alongside him were Jehoram's brethren. Although those other sons received large presents of silver and gold and valuable things, together with fortified cities in Judah, the kingdom itself was given to Jehoram because he was the firstborn. This shows that the firstborn was heir to the throne, connecting birthright with blessing and inheritance.
Jehoshaphat's kindly but misdirected alliance with Ahab and the house of Ahab corrupted his household. That corruption came through Jehoram's marriage to an idolatrous wife, the daughter of wicked Ahab and Jezebel. Jehoram was a wicked king who was led away by that idolatrous wife.
The beginning of Jehoram's reign in his father's place was marked by his destruction of all his brethren. His career as king was short. His end was fearful.