Eisav and Yishmael are frequently - erroneously - referred to as "brother" and "cousin" by some Jews, but this is incorrect. The Torah itself bears witness that Hashem Yitbarach made His covenant with Yitzchak, but not with Yishmael (Gen. 17:19-20), and with Ya'aqov, but not with Eisav (Gen. 28:13). That's why we refer to HKB"H as the God of Avraham, Yitzchak and Ya'aqov.
The Ramchal explains the relationship between Israel and the Nations in Derech Hashem like this:
...Avraham's tree consisted of 600,000 main branches. These were the individuals who left Egypt, and it was to them that the Torah was given and the land of Israel divided. Every Jew subsequently born is considered to be an element and descendant of one of these primary branches.
It was to these 600,000 original Jews that the Torah was given. When this occurred, the tree was said to have attained maturity.
At this time, God also gave the nations a last chance. In His mercy he had suspended their final judgment until the time that the Torah was given [with the revelation at Sinai]. He then offered the Torah to every nation, giving them the opportunity to accept it.
If any nation would have accepted the Torah, it would have elevated itself from is lower state. As it was, none of them desired the Torah, and their judgment was therefore sealed completely. The gate was permanently closed, never again to be opened.
It still remained possible, however, for any individual to convert to Judaism. In this manner, he could still include himself in Avraham's tree of his own free will.
The decree, however, was not that the other nations should be destroyed. It only meant that they would have to remain on the lower level that we have discussed. This lower state would never have been meant for man if Adam had not sinned. It only came into being in the first place as a result of his sin.
These nations still have the human aspect, blemished though it may be, and God desired that they should at least have a counterpart of what was actually appropriate for all mankind. He therefore granted them a Divine Soul (neshamah) somewhat like that of the Jew, even though it is on a much lower level. They were likewise given commandments, through which they could attain both material and spiritual advantages appropriate to their nature. These are the seven [universal] commandments given to the children of Noach.
All this had been arranged from the beginning of creation, prepared for the contingency that man would sin. In this respect, it is like other harmful things and punishments, which were also created conditionally, as our Sages teach us.
In the World-to-Come, however, there will be no nation other than Israel.
The souls of righteous gentiles will be allowed to exist in the Future World, but only as an addition and attachment to Israel. They will therefore be secondary to the Jew, just as a garment is secondary to the one who wears it. All that they attain of the ultimate good will have to be attained in this manner, since by virtue of their nature they can receive no more.
When the world was divided into seventy nations, God appointed seventy [directing] angels as Officers (Sarim) in charge of these nations, to watch over them and attend all their needs.
Thus, God does not oversee these nations except in a general manner. It is each one's directing angel who takes care of the details, through the power that God gives it for this purpose. God thus told Israel (Amos 3:2), "Only you have I known among all the families of the earth."
...God thus made the rectification and elevation of all creation totally dependent on the Jews. To the extent that this can be expressed, we can say that He subjugated His Providence to them. Through their deeds, they can cause [His Light] to shine forth and have influence, or, on the other hand, hold it back and conceal it.
The deeds of the other nations, on the other hand, do not add to or subtract from the state of creation, nor do they cause God to reveal Himself or withdraw. All they can do is bring about their own gain or loss, and strengthen or weaken their own directing angel.
Although God does not involve Himself expressly in the direction of the nations in detail, it is possible that He should exercise His Providence over them when it is required for the sake of Israel. This may be because of a single Jew, or for a number of them. (Derech Hashem, Part II: Providence, Chapter 4: Israel and the Nations)Given this, I don't see how it will be possible for either Eisav, or Yishmael to "do teshuvah" to the extent of rectifying and elevating themselves to the level of the offspring of Avraham Avinu at the End of Days. The time for this ended and it is now only possible for individuals, but even this is coming to an end with the arrival of Mashiach Tzidkeinu.
Furthermore, it exposes the error currently spreading around the world that the gentiles have a crucial role to play in bringing about the redemption of the world or that they are somehow our partners in same.