Why did Jacob insist that Joseph's sons were his?

Question

In Genesis, Before Jacob dies, he has a conversation with his son, Joseph.

Genesis 48:1-6

Some time later Joseph was told, “Your father is ill.” So he took his two sons Manasseh and Ephraim along with him. 2 When Jacob was told, “Your son Joseph has come to you,” Israel rallied his strength and sat up on the bed. 3 Jacob said to Joseph, “God Almighty[a] appeared to me at Luz in the land of Canaan, and there he blessed me 4 and said to me, ‘I am going to make you fruitful and increase your numbers. I will make you a community of peoples, and I will give this land as an everlasting possession to your descendants after you.’ 5 “Now then, your two sons born to you in Egypt before I came to you here will be reckoned as mine; Ephraim and Manasseh will be mine, just as Reuben and Simeon are mine. 6 Any children born to you after them will be yours; in the territory they inherit they will be reckoned under the names of their brothers.

What is up with this passage? Why is Jacob insisting that Joseph's sons were his?

Answer 1

This appears to be essentially a bit of pragmatism. The foremost heir would inherit a double share of the father's inheritance, (this is what Elisha was referring to in 2 Kings 2:9,) and Joseph had been given this honor.

Since the inheritance was to be passed down throughout generations, and Joseph had two sons, Jacob essentially said "we'll do it like this: each of your two sons gets a full share, and that takes care of your double share."

Answer 2

In patriarchal times, inheritance was [almost exclusively] passed-on to actual offspring of the family.

Jacob in essence is adopting Joseph's two sons so that he can bless them and "legally" pass-on their inheritance.