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