The Torah says that HaShem blessed Abraham with Bakol (literally "everything"). One understanding in the Talmud is that this means that although Abraham had already been blessed with Issac, his blessings reached an apex with the birth of a daughter named Bakol. Tosafos ask, if this is true, then according to the opinion that a Noahide may marry his sister, why Issac did not marry Bakol. In the first answer, Tosafos say that since Bakol was too young at the time that Issac was looking for a wife, Abraham did not consider taking her as a wife for his son. At the time of the Binding of Issac, when Issac was thirty-seven years old, Rebecca, a reincarnation of Sarah, was born, married Issac three years later, when Issac was forty. The question then arises why Issac married his young cousin Rebecca, if Rebecca herself was only three years (and a month) old at the time of their marriage, and it seems that Abraham would not have wanted his son to marry a pre-pubescent girl. This seems to be a proof to the opinion recorded in Tosafos that Rebecca was actually fourteen years old at the time of her marriage, not three. In describing Rebecca, the Torah states, "And the maiden [na'arah] was very good in appearance; [she was] a virgin, whom no man had known her". Since the term Na'arah usually refers specifically to a girl over the age of twelve, this implies that Rebecca was "of age" already. Furthermore, the Torah seems to be praising Rebecca for her chastity, which implies that she had reached an age when there is normally a desire to engage in sexual relations, and yet she remained chaste because of her modesty. This age is obviously older than three years old.
Rabbi Shimon Schwab (1908-1993) explains in the name of Rabbi Michael HaCohen Forshlager of Baltimore (a student of the Sochachover Rebbe, the Avnei Nezer) that both opinions are actually correct, and his explanation can incidentally answer the question as to why Issac could have married such a young girl as Rebecca, but not Bakol. He explains that physically, Rebecca was a fourteen-year-old young bride at the time of her marriage, but spiritually she was a reincarnation of Sarah, who had only died three years prior. Only once Sarah's soul left her at the time of the Binding of Issac, did Rebecca become a reincarnation of Sarah, and thus at that time she was "born again." Rabbi Schwab adds that this explains why Rashi explained that when the Torah says, "Issac brought her [Rebecca] into the tent of Sarah, his mother", the Torah means to say that Rebecca was Sarah; this is because they shared a soul.
In a second answer, Tosafos say that Bakol was a daughter of Hagar, not Sarah, thus she was not worthy of marriage to Issac. Rabbi Yaakov Emden (1697-1776) explains that even though Hagar converted to Judaism, it was still not fitting that Issac, Abraham's son through his main mistress, Sarah, should marry Bakol, who was a daughter of Abraham through his maidservant, Hagar. Furthermore, writes Rabbi Yaakov Emden, when one marries a girl, he is supposed to examine his prospective bride's brothers in order to see if she is worthy for marriage. If Bakol was a daughter of Hagar, then her brother was Ishmael, who had already been established as a wild hunter, whose full sister would be completely unsuitable for marriage.
However, writes Rabbi Yaakov Emden, the original question of Tosafos as to why Issac did not marry Bacoll does not even begin as a question. He writes that Abraham certainly did not have the status of a Noahide once he circumcised himself (which predated Isaac's birth by a year). He also suggests that perhaps Abraham lost his status as a Noahide and became a Jew even before his circumcision, that is, at the time that he began engaging in Torah study. In this detail, Rabbi Yaakov Emden is consistent with his opinion elsewhere where he explained that the betrothed women with whom Esau engaged in adultery were converts from the time of Abraham. This answers the question of Tosafos who ask with which women had halachikly recognizable marriages in the time of Jacob and Esau that Esau could have been vilified for committing adultery.
Rabbi Yehoshua Leib Diskin of Brisk/
that Bakol was a daughter of Sarah, but she could not have married Issac
because she died at the same time as her mother Sarah died,
which was before Abraham began looking for a match for Issac. A proof to this
is that when Abraham came to Hebron to lament the loss of his wife, the chaf
in the word ve'livkosoh ("and to cry for her") is written
small in the Torah, as
if it should be omitted and read velivita ("and for her
daughter"), meaning that Abraham mourned for his daughter, Bakol, also.
However, in a letter dated 1979, Rabbi Moshe Feinstein (1895-1986) questions whether
the Maharil Diskin ever said this explanation.
Firstly, the order of the verses in the Torah seems to imply that the Bakol
lived after Sarah because the verse, which is expounded, is written immediately
after the passage describing Abraham's burial procedures for his deceased wife.
Secondly, if Bakol had died during the lifetime of Abraham, how can that have
been considered a blessing for Abraham? One can also ask that if Bakol died
before Issac began searching for a spouse, then why Tosafos asked that Issac
should have married her. Jerusalem
According to the explanation that Bakol died at the same time as Sarah, it is also difficult to understand a Midrash, which explains Abraham marriage to Keturah after Issac's marriage to Rivkah. The Midrash says that Abraham's actions are a fulfillment of the verse, "Sow your seed in the morning, but do not rest your hand in the evening". Rabbi Yehudah Aryeh Leib Rothenberg/Alter (1847-1905) explains that this seems to say that the reason why Abraham married Keturah is simply to raise children in his older years as he did in his younger years, not in order to fulfill his commandment of procreation, because he already fulfilled it with his daughter Bakol. Abraham fulfilled his obligation to procreate according to both Beis Shammai and Beis Hillel by having two sons, Issac and Ishmael, according to Beis Shammai and a son and daughter, Issac/Ishmael and Bakol. However, if Bakol had already died by that time, then Abraham obviously married Keturah in order to fulfill his commandment of procreation, which Bais Hillel says requires one to father both a son and a daughter—even if one child, heaven forbid, dies, the obligation returns. Perhaps one can answer that Abraham subscribed to Beis Shammai's opinion that two sons is enough to fulfill the obligation or that even according to Beis Hillel, Abraham had another daughter. The Talmud and Midrash both mention based on a verse in Song of Songs that Abraham had a daughter named Nediv.
The Tosefta records the opinion of Rabbi Meir, the husband of the seemingly egalitarian Bruria. Rabbi Meir said that Abraham's blessing was that he did not have any daughters. The simple understanding of this is based on a Talmudic dictum, "Woe onto he whose children are female" which implies that having daughters is not a blessing, but rather a curse. Rabbi Yaakov Culi (d. 1732) explains that according to Rabbi Meir, having a daughter would have been a curse for Abraham because he would have been forced to marry his daughter to a native Canaanite, so Abraham was blessed by not birthing daughters. Nachmanides (1194-1270) explains that even if Abraham would have had a daughter and sent her to his family east of the Jordan to get married, then she would have adopted their idol worshipping practices, which would have been a curse for Abraham. Furthermore, Nachmanides understands (unlike the second answer of Tosafos) that Bakol was a daughter of Sarah, and Abraham would not have wanted his daughter from Sarah to leave the
Land of Israel, just like Issac never left the . Because of these reasons, Rabbi
Meir reasoned that Abraham having a daughter would have been more painful for
himself than him not having fathered a girl. However, according to Rabbi Yehuda's
opinion that Abraham did father a girl named Bakol, one can say that it was not
a curse for him because she did not live to a marriageable age, so Abraham
never had the pain of having to take an idolatrous son-in-law or send his
daughter out of land of Israel .
Rabbi Yehuda reasoned that since Abraham was blessed with
"everything", he must have even had a daughter, whose name, Acheirim
say, was Bakol. Rabbeinu Bachya ben Asher (d. 1340) adds
that usually a man desires to father both girls and boys, so according to the
opinion that Abraham had a girl, he fulfilled this desire. Israel
 Genesis 24:1
 Bava Basra 116b
 See Eitz Yosef to Bava Basra 116b
 To Bava Basra 141a
 Sanhedrin 58b
 Perhaps this is because marrying a pre-pubescent girl, who cannot yet conceive, delays the arrival of Moshiach (Niddah 13b) or because when the Talmud said (Pesachim 113a) one should marry off one's daughter as quickly as possible, even to one's slave, that was said only once she becomes "of age" and not before.
 Pirkei D'Rabbi Eliezer, Chapter 31, Yalkut Shimoni, beginning of Toldos
 Genesis Rabbah §57
 Genesis 35:20
 Pirkei D'Rabbi Eliezer Chapter 32, Seder Olam, and Tosafos to Yevamos 61b
 Tractate Sofrim, end of Chapter 21
 Yevamos 61b
 Genesis 24?:16
 Maayan Beis ha-Sho'eva to Genesis 25:20
 See Leviticus Rabbah §20, Midrash Tanchuma end of Vayera, Pirkei D'Rabbi Eliezer Ch. 32, which say that Sarah died around the time of the Binding of Issac
 Genesis 24:67
 To Bava Basra 141a
 Tosafos HaShalaeim (Genesis 16:2) write that Bakol was indeed the daughter of Sarah, not Hagar. They explain that so is evident from an implication in a scriptural passage in which, before the conception of Isaac, Sarah tells Abraham (ibid.) that G-d has stopped (atazar) her from birthing. In a similar story, Rebecca prayers (Genesis 25:21) to become pregnant because she was barren (akar). From the fact that by Sarah the Torah says stopped and by Rebecca says she was barren (or uprooted from having children according to a literal transaltion of akar), Tosafos HaShaleim infer that Sarah did give birth previously, i.e. to Bakol, and was merely telling Abraham that she had failed to sire him a son. While, on the other hand, Rebecca did not have any children at all because she was completely barren.
 Hagahos Ya'avetz to Bava Basra 141a
 Yevamos 100b
 Bava Basra 110a. This is learned out from the fact that the Torah (Exodus 6:23) tells that Aharon HaKohen married Elisheva, daughter of Aminadav and that she was the sister of Nachshon. The Talmud understands that Aharon HaKohen decided she was a suitable wife because her brother was the prince of the tribe of Judah.
The halachik status of the forefather is an issue which is beyond the scope of this essay.
 Hagahos Ya'avetz to Tosafos to Bava Basra 16b
 Tosafos to Bava Basra 16b
 Kisvei Maharil Diskin
 Shai L’Torah (Parsha Chayei Sarah) writes in the name of Rabbi Chaim Kanievsky that a recently-published early medieval source, known as Mincha Bellulah, writes the exact same explantion as Rabbi Diskin. This explanation is also found in the introduction to responsa Binyan Shlomo (new ed.) from Rabbi Shlomo HaKohen of Vilna (1828-1905) in the Torah novellae written by the father of the author (Parshas Chayeh Sara).
 Genesis 23:2
 Iggress Moshe, Volume 4 of Orach Chaim, §6
 Genesis 25:1
 Genesis Rabbah §61:3
 Ecclesiastes 11:6
 Sefas Emes to Parshas Chayei Sarah
 Yevamos 61b
 Yevamos 62a
 See "Avrohom and Pru U'revu - What About His (Not So Well-Known) Daughter?" by Rabbi Moshe Heigh
 Chagigah 3a
 Song of Songs Rabbah §7:2
 Kiddushin 5:14
 Kiddushin 82b, see also Sanhedrin 100b which explains why daughters are such a "curse."
 Me'Am Loez To Genesis 24:1
 Ramban to Genesis 24:1
 Rabbeinu Bachaya to Genesis 24:1