Genesis 48

It is apparent that Joseph received the birthright which includes that right of double an inheritance as Jacob adopts Joseph's sons a his own saying -

"...now thy two sons, Ephraim and Manasseh, which were born unto thee in the land of Egypt before I came unto thee into Egypt, are mine; as Reuben and Simeon, they shall be mine.

6 And thy issue, which thou begettest after them, shall be thine, and shall be called after the name of their brethren in their inheritance."  (Old Testament | Genesis 48:5 - 6)

Thus there now seems to be 13 tribes of Israel.  The importance of this becomes more apparent in the JST where we read -

6 And thy issue which thou begettest after them, shall be thine, and shall be called after the name of their brethren in their inheritance, in the tribes; therefore they were called the tribes of Manasseh and of Ephraim.
7 And Jacob said unto Joseph, When the God of my fathers appeared unto me in Luz, in the land of Canaan; he sware unto me, that he would give unto me, and unto my seed, the land for an everlasting possession.
8 Therefore, O my son, he hath blessed me in raising thee up to be a servant unto me, in saving my house from death;
9 In delivering my people, thy brethren, from famine which was sore in the land; wherefore the God of thy fathers shall bless thee, and the fruit of thy loins, that they shall be blessed above thy brethren, and above thy father’s house;
10 For thou hast prevailed, and thy father’s house hath bowed down unto thee, even as it was shown unto thee, before thou wast sold into Egypt by the hands of thy brethren; wherefore thy brethren shall bow down unto thee, from generation to generation, unto the fruit of thy loins forever;
11 For thou shalt be a light unto my people, to deliver them in the days of their captivity, from bondage; and to bring salvation unto them, when they are altogether  bowed down under sin."  (JST - Genesis 48:6-11)

I will note the great responsibility of these two birthright tribes in verse 11 in that they are to deliver the people and bring salvation to the people in the latter days.

We learn why the posterity of Ephraim was to take the lead in this great work for as -

Joseph brought them out from between his knees, and he bowed himself with his face to the earth.

13 And Joseph took them both, Ephraim in his right hand toward Israel's left hand, and Manasseh in his left hand toward Israel's right hand, and brought them near unto him.
14 And Israel stretched out his right hand, and laid it upon Ephraim's head, who was the younger, and his left hand upon Manasseh's head, guiding his hands wittingly; for Manasseh was the firstborn.
15 ¶ And he blessed Joseph, and said, God, before whom my fathers Abraham and Isaac did walk, the God which fed me all my life long unto this day.
16 The Angel which redeemed me from all evil, bless the lads; and let my name be named on them, and the name of my fathers Abraham and Isaac; and let them grow into a multitude in the midst of the earth.
17 And when Joseph saw that his father laid his right hand upon the head of Ephraim, it displeased him: and he held up his father's hand, to remove it from Ephraim's head unto Manasseh's head.
18 And Joseph said unto his father, Not so, my father: for this is the firstborn; put thy right hand upon his head.
19 And his father refused, and said, I know it, my son, I know it: he also shall become a people, and he also shall be great: but truly his younger brother shall be greater than he, and his seed shall become a multitude of nations.
20 And he blessed them that day, saying, In thee shall Israel bless, saying, God make thee as Ephraim and as Manasseh: and he set Ephraim before Manasseh."  (Old Testament | Genesis 48:12 - 20)

The responsibility of Ephraim then was to take the lead in the work of salvation.