Guardians teach their kids to regard their senior kin, it would accordingly be rude to the guardians to slight one's senior sibling.8 The firstborn is likewise meriting regard since he is considered fairly answerable for his kin. The Midrash explains9 this is the reason Reuben spared Joseph from unavoidable passing because of his siblings, proposing rather that he be tossed into a pit (while expecting to return later to spare him).10 For he said to himself, "I'm the firstborn; the fault [of Joseph's death] will fall just on me."
Guardians teach their kids to regard their senior kin, it would accordingly be rude to the guardians to slight one's senior sibling