“[Reuben] was the firstborn, but because he defiled the bed of his father, his right as firstborn was given to the sons of Joseph the son of Israel […]”
~1 Chronicles 5:1

The “sons of Israel” totaled 12, but the first born was supposed to receive double the inheritance portion of the others.
Israel’s first-born, Reuben, slept with his father’s concubine, thereby forfeiting his right to his double portion of inheritance (Ge. 35:22).
Joseph was Israel’s favorite son, long believed to have died before reappearing in Israel’s life as savior and provider for their entire household (Ge 45:25–46:4).
Also, he was the first-born son of his favorite wife, Rachel (Ge. 29:30; 30:22-24).
Joseph had two sons, Manasseh and Ephraim, and Israel prophesied that the younger would dominate the older (Ge. 48:13-20; Jos. 14:4).
The distribution of tribal land was to be divided among twelve tribes (Ge. 49:2-28).
This significant number worked out well when we take into account that the sons of Levi, whose jobs were directly related to the worship of Jehovah, would not receive their own territory (Nu. 18:20).
Therefor, there were thirteen total tribes of Israel, twelve of which had territories.
Israel’s prophecy that the tribe of Ephraim would grow more than his brother’s began to materialize in chapter seven of this week’s reading.
In fact, Ephraim grew so large as a tribe that it eventually came to represent the northern nation of Israel, after the original nation divided in two (Ho. 13:1).
From Ephraim came both spiritual people inclined to serve God, while other Ephraimites became apostates (1 Ki. 12:25-30; 2 Chron. 15:9).
The point I wish to highlight from this reading is that while God can decide our future in general terms, as he did with the tribes of Reuben, Manasseh and Ephraim, it is up to us as individuals to decide whether or not we want to serve him.