But Joseph was upset when he saw that his father placed his right hand on Ephraim’s head. So Joseph lifted it to move it from Ephraim’s head to Manasseh’s head. “No, my father,” he said. “This one is the firstborn. Put your right hand on his head.” But his father refused. “I know, my son; I know,” he replied. “Manasseh will also become a great people, but his younger brother will become even greater. And his descendants will become a multitude of nations.”

Genesis 48:17-19

The scepter will not depart from Judah, nor the ruler’s staff from his descendants, until the coming of the one to whom it belongs, the one whom all nations will honor. 
Genesis 49:10

• Jacob put Ephraim and Manasseh in replace of Reuben and Simeon. He did not disinherit Reuben and Simeon, but prioritized Joseph's two sons’ standing in their inheritance. “The adoption practice and formula here are very similar to those attested in the code of Hammurabi. Additionally, one Ugaritic text features a grandfather adopting his grandson. In one sense this adoption could be seen as the means by which Joseph is given the double portion of the inheritance due to the firstborn, since two of his sons receive shares from Jacob’s inheritance.” IVP Bible Background Commentary
• “Now I am claiming as my own sons these two boys of yours, Ephraim and Manasseh, who were born here in the land of Egypt before I arrived. They will be my sons, just as Reuben and Simeon are.”
  Genesis 48:5
• Isaac received inheritance over Ishmael, Jacob over Esau, Ephraim over Manasseh. Though ancient culture usually allotted the most inheritance to the oldest, exceptions were sometimes made for various reasons.
• Blessings or curses pronounced by the patriarch of the family were taken very seriously and considered binding by the family, but cannot be considered as always a prophetic announcement from God. 
• The scepter will not depart from Judah, nor the ruler’s staff from his descendants, until the coming of the one to whom it belongs, the one whom all nations will honor.
  Genesis 49:10
• God sometimes works within culture to speak to us, but is never bound or boxed in by cultural expectations. 

Be ready to listen to God in whatever way He speaks to us, in ways we sometimes do not expect.