Ge 48:1 Now it came to pass after these things that Joseph was told, "Indeed your father is sick"; and he took with him his two sons, Manasseh and Ephraim.

 2 And Jacob was told, "Look, your son Joseph is coming to you"; and Israel strengthened himself and sat up on the bed.

 3 Then Jacob said to Joseph: "God Almighty appeared to me at Luz in the land of Canaan and blessed me,

 4 "and said to me, 'Behold, I will make you fruitful and multiply you, and I will make of you a multitude of people, and give this land to your descendants after you as an everlasting possession.'

 5 "And now your two sons, Ephraim and Manasseh, who were born to you in the land of Egypt before I came to you in Egypt, are mine; as Reuben and Simeon, they shall be mine.

 6 "Your offspring whom you beget after them shall be yours; they will be called by the name of their brothers in their inheritance.

 7 "But as for me, when I came from Padan, Rachel died beside me in the land of Canaan on the way, when there was but a little distance to go to Ephrath; and I buried her there on the way to Ephrath (that is, Bethlehem)."

 8 Then Israel saw Joseph's sons, and said, "Who are these?"

 9 And Joseph said to his father, "They are my sons, whom God has given me in this place." And he said, "Please bring them to me, and I will bless them."

 10 Now the eyes of Israel were dim with age, so that he could not see. Then Joseph brought them near him, and he kissed them and embraced them.

 11 And Israel said to Joseph, "I had not thought to see your face; but in fact, God has also shown me your offspring!"

Here, Moses is showing his original Israelite readers as to why the tribes of Ephraim and Manasseh came to inherit a portion of the land of Promise ---  Jacob adopted them as his own:  "... now your two sons, Ephraim and Manasseh, who were born to you in the land of Egypt before I came to you in Egypt, are mine, as Reuben and Simeon, they shall be mine...." 

Since the two were adopted by Jacob as his own sons, they, therefore, received an equal standing with the children of Israel. Being adopted, they are not to be viewed as minor sons or lesser sons. No, they are sons "as Reuben and Simeon." Reuben and Simeon were the eldest son and the second son of Jacob respectively. As sons of equal standing with the rest, they (Ephraim and Manasseh)  too would inherit the land that God promised to Jacob as an eternal possession.

Reuben lost the privileges of being the firstborn. As the firstborn, he should have been the head of the family and an inheritor of a double blessing. Reuben had sexual relations with Jacob's concubine.

Ge 35:22 And it happened, when Israel dwelt in that land, that Reuben went and lay with Bilhah his father's concubine; and Israel heard about it.

Reuben lost the right and the privilege to be the head of the family, it went instead to Judah. From Judah, the Ruler or the great King, the Messiah, would come. Because of the violence of Simeon and Levi (Ge 49:34:25) which caused great shame and distress to their father they too were bypassed in the right and privilege to be head. Instead, it went to the fourth son, Judah.

Reuben instead of receiving a double portion of the land inheritance received only a single portion. One of his portions was given to Joseph which made Joseph an inheritor of two portions. It was then given to Ephraim and Manasseh.

Levi also lost the blessing of receiving a land inheritance. Thus, the tribe of Levi inherited no land later on.  As for Simeon, you can read Jacob's curse on him along with Levi in Gen. 49:6-7.