Why did the Torah say that Timna was Lotan's sister, rather than list her with Se'ir's sons?
Rashbam and Ramban #1: It is customary to attribute a woman to her oldest brother. 1
Ramban #2: To teach us that she was a full sister of Lotan, but not a full sister of Se'ir ha'Chori's other sons (who were from a different mother).
Ramban #3: She was Lotan's sister only from her mother's side, but was not the daughter of Se'ir ha'Chori. 2
See for example, the last Pasuk in Toldos (28:9); and Shemos 15:20 (Rashbam). See also Ramban.
Of what importance is it to know that Timna was Lotan's sister?
Refer to 36:12:1:1.
Why does our verse say "va'Yihyu;" verses 36:23-26 begin with "v'Eleh;" and verses 36:27-28 begin with "Eleh"?
Ha'amek Davar: The Torah hints to Hashgachah; division among Bnei Se'ir enabled Esav to settle there [and conquer the land]. Some Bnei Se'ir consented that Esav's seed come. Bnei Lotan agreed; they gave Lotan's sister to marry [Elifaz]! "V'Eleh" refers to others who consented; from Tziv'on, they let Aholivamah marry Esav! Verses 36:27-28 begin with "Eleh;" the families of Etzer and Dishan did not consent. 1
Ha'amek Davar (to Devarim 2:22): Devarim 2:12 says that Bnei Esav inherited from the Chori, and then wiped them out and settled there. Verse 22 reverses the order (i.e. they wiped them out, then inherited them)! Bnei Aholivamah were from the Chori (maternally), so the Chori let them settle there, and afterwards, Bnei Esav destroyed the Chori. The Chori did not let Esav's other sons settle there; they could not settle there until they destroyed the Chori.
