score:5
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."
Upvote:2
In patriarchal times, inheritance was [almost exclusively] passed-on to actual offspring of the family.
Jacob in essence is adopting Joseph's two sons so that he can bless them and "legally" pass-on their inheritance.