Why did Hashem refer to Eisav as Yisrael's brother?
Ramban: Because the lineage of Yisrael stems from Avraham, all of whose offspring are called 'brothers', since they were all circumcised. 1 This is why it says "do not hate an Edomi, for he is your brother" (23:8).
Ramban: With the exception of the sons of Avraham's concubines, Yishmael and B'nei Keturah, whom Hashem precluded when He said to Avraham (in Bereishis 21:12) "Ki be'Yitzchak Yikarei L'cha Zara". See Nedarim 31a, which precludes Eisav from "be'Yitzchak", and not all of Yitzchak? See Rav Chavel's footnotes.
Why does the Torah insert the words "ha'Yoshvim be'Se'ir"?
Rashbam: Because it was only the B'nei Eisav who lived in Se'ir who offered to sell them food and water; those who lived in Edom responded to their request by coming out to meet them with the sword. 1
Rashbam: See Bamidbar 20, 21.
What is "Venishmartem Me'od" referring to?
Rashi: It is referring to waging war against Se'ir, as the following Pasuk explains.