Chazal in Medrash Rabboh on Bereishis interpret the posuk that Avrohom moved away from Lot because the people were talking about the fact that Lot slept with his daughters. At that point the nations still guarded themselves from illicit marriages. The Ramban says nothing about this. I have no idea where you got this idea that his daughters were muttar to him. They were his daughters from his wife.
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