3 And Jacob said unto Joseph, God Almighty appeared unto me at Luz in the land of Canaan, and blessed me, 4 And said unto me, Behold, I will make thee fruitful, and multiply thee, and I will make of thee a multitude of people; and will give this land to thy seed after thee for an everlasting possession. 5 And 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.
Presumably Joseph only brought his sons to receive a blessing, but in an unexpected turn of events, now they are being adopted by their grandfather! This is hardly an adoption in the traditional sense, though, Jacob is about to die and is not going to raise these two boys in place of Joseph. Rather, he is saying that they will receive of his inheritance, as if they had been born among his immediate sons. They will be counted among the twelve tribes of Israel. And this is why, when Joshua will later divide the land of Canaan among the twelve tribes, there will not be just one land for Joseph’s descendants, but two: one for Ephraim and one for Manasseh.
Which brings up the question of “just what are the twelve tribes of Israel?” The answer to that is a bit inconsistent. Jacob had twelve sons naturally, and here he is said to be adopting two more. In Deuteronomy 27:12-13, the tribes are listed under the names of Jacob’s twelve biological sons, but when the land of Canaan was divided by Joshua, Joseph was replaced with Ephraim and Manasseh, and Levi was not given any land, only specific cities and the temple in Jerusalem. So, in a sense, Jacob had either twelve, thirteen, or fourteen branches, depending on which aspect of Israel you are talking about.
It is also worth noting that while Jacob said Joseph would retain any further sons within his own house, we never receive any indication that Joseph did have any other sons. And if he didn’t, this would explain why there wasn’t an additional land of “Joseph” parceled out when the Israelites came into Canaan.