Why does Rashi explain that Oholivamah was the result of an adulterous relationship between Tzivon and his daughter-in-law?
(36,2) “Eisav took his wives from the daughters of Canaan - Adah the daughter of Eilon the Hittite, and Oholivamah the daughter of Anah, the daughter of Tzivon the Hivite.”
Our posuk says that Eisav married Oholivamah who was apparently the daughter of Anah since his wife had given birth to her. But the Torah also writes that she was the daughter of Tzivon who was Anah’s father. Rashi resolves this obvious contradiction by explaining that Tzivon slept with his daughter-in-law and thus was the real father of Oholivamah.
Later on in posuk 20, the Torah writes that Anah was the son of Seir, Tzivon’s father, but in posuk 24 it writes that Anah was the son of Tzivon. Rashi resolves that problem by explaining that Tzivon slept with his mother, the wife of Seir, and thus was the real father of Anah.
The question everyone asks is why Rashi did not explain both cases in the same way, by saying that also Seir slept with his daughter-in-law, the wife of Tzivon, and gave birth to Anah, thus appearing to be the son of Tzivon but who was in reality the son of Seir. Alternatively, he could have explained that also Anah slept with his mother, the wife of Tzivon, and gave birth to Oholivamah, thus appearing to be the daughter of Tzivon but who was in reality the daughter of Anah.
But we can explain that Rashi’s explanations are based on two rules. The first rule is that the Torah never writes something which is an obvious contradiction unless it is necessary to do so. The second rule is that we try to avoid explaining that someone slept with his mother because such a thing is very rare and thus unlikely, as we learn from the gemora in Sanhedrin and the gemora in Yevamos.
Therefore, Rashi could not have explained that Seir slept with his daughter-in-law, because if so it would not have been necessary for the Torah to write the contradictory statement that Anah was both the son of Seir and the son of Tzivon. It would have been sufficient just to write that Anah was the son of Seir - firstly because he really was his son, but also because even according to what the world thinks, that he was the son of Tzivon because his wife gave birth to him, nevertheless it would have been correct to call Anah the son of Seir since grandchildren are considered to be like one’s children, and are often referred to as such. Thus, since the Torah considered it necessary to write that Anah was also the son of Tzivon, Rashi was forced to explain that Tzivon slept with his mother.
But with Oholivamah there are no problems if we explain that Tzivon had relations with his daughter-in-law. Because in this case we cannot ask that if so why did the Torah not just write that she was the daughter of Tzivon, since she really was his daughter, and even if she appeared to the world to be the daughter of Anah it would have been fitting to call her the daughter of Tzivon, because grandchildren are sometimes referred to as children, and Anah was the son of Tzivon, as Rashi wrote above.
We cannot ask this here because the Torah has to take into account what the world thinks completely, and the world thinks not only that she was the daughter of Anah since his wife gave birth to her, they also think that Anah was the son of Seir since the wife of Seir gave birth to him, because Tzivon had relations with his mother as we explained above. Therefore, if the Torah had only wrote that she was the daughter of Tzivon this would not fit with what the world thinks, and so it was absolutely necessary to write that she was also the daughter of Anah.
Thus, since we are able to explain that he slept with his daughter-in-law, we would never choose to explain that he slept with his mother, as we explained earlier, and for this reason Rashi explained the way he did.