“But we know that the “oral tradition” began after the return to the land.”
No, it was given to Moshe at Mt. Sinai.
But I concur there are major issues with it; the written versions are not clearly recorded, conflict, and were prepared much later.
Hence, the two reasonable camps.
Arguing in your favor, would be that the narrower interpretation (half-sibling) would help explain the problem with fertility, whereas Abraham had no problem conceiving with Hagar.
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Nothing “oral” was given at sinai, that is Pharisee blasphemy. That idea came in the 2nd temple period.