I noticed yesterday that Rashi (implicitly) rejects the idea that Jacob's twelve sons all married their sisters. Writing about Potiphar's wife (I don't have a chumash, or I'd give you the verse) he says that she acted for the sake of Heaven. She wanted to have Joseph's children because she saw, through astrology, that it was the divine will for Joseph's children to come from her.
This, Rashi, concludes, was fulfilled through her daughter Asneth. In other words, Rashi considered Asneth a bona fide Egyptian, and not Dina's daughter.