God-ordained Metaphysical Union
These texts indicate a metaphysical union of man and woman as husband and wife. The mystery of becoming "one" is understood to reflect the nature of a complementary joining of the sexes into a new familial unit, as God intended it be, ordinarily attended by the blessing of progeny. [Note: I have not found the key words in these texts to lend greater insight than the translations.]
The "leaving" is coterminous with the recognition of the nuclear family (begun by husband-wife). The family is the basic unit of human organization in the created order. Thus, matters of household authority, economy, and relations are shouldered by each successive generation. The recognition of such independence of the family is a natural right, susceptible to the situational banes and blessings of sin and grace.
Settling a Family Unit
Proximity or multifamily living situations do not negate the family unit, though they may infringe upon their functions. The settlement of Israel by tribes in the land of Canaan provides a geographic picture of familial proximity, but the facts of inheritance, separate tents, separate cities, etc. all evidence the fact of the spread of families.
While, the command to honor one's father and mother remains throughout one's life as a principle guideline, when a man and wife begin a family, scripture teaches [both the Hebrew and New Testament scriptures] that the two are now distinctly separated from their mothers and fathers. There are new roles of authority, economy, and the raising of a new family to be taken up by the new pair.
Walking in Honor and as a New Household
Between letting children be married leave home, and honoring ones parents, we have the basic principles upon which the circumstances of our successive generations are worked out whether in obedience or disgrace. In general, successive generations may "walk" in the fear of the LORD and receive blessings on their generations; or, they may walk in a variety of other idolatrous and adulterous ways to the condemnation of their generations.
This is all very general, but we are talking about the basic letting of a husband and wife to govern their household; or the hindering of that basic succession. As to whether it is sinful for a newly married couple to live on property with parents, that is a question of the roles and economy taken up - are they free from the meddling of their affairs? And the parents also? Each must be careful of their responsibilities in the agreement and to not overwrite the established order, which God has made.
Further clarity on Isaac's position in regards to the ethics of his situation in his mother tent is needed from historical information. That information would be illustrative of the application of these principles (of honor and leaving). [Perhaps I can bring that info back later, unless someone else supplies it.]
