According to Breishit 28:9, Esav married Machalat, "daughter of Yishmael son of Avraham, sister of Nevayot". Why does the verse mention both Machalat's father and brother?
The midrash's answer is that Yishmael died right around the time of Machalat's marriage. Thus, immediately beforehand she was part of the family run by Yishmael, and immediately afterwards she was part of the family run by Yishmael's oldest son, Nevayot. Thus she is mentioned in relation to both Yishmael and Nevayot.
If so, then Esav's marriage took place at the same time as Yishmael's death. It seems from chapter 28 that Esav's marriage took place shortly after Yaakov stole the blessings and fled home. How old was Yaakov when this happened? Yishmael died at age 137. Yitzchak, who was 14 years younger than Yishmael (they were born to Avraham at ages 86 and 100), was age 123 at the time. Yitzchak fathered Yaakov at age 60, thus Yaakov was age 63 when he left home.
How old was Yaakov when he arrived in Haran? When Yaakov arrived in Egypt he was 130 years old. Yosef was 39 years old at the time (30 years old when Pharaoh promoted him, 7 years of plenty, 2 years of famine.) Thus Yaakov was 91 when Yosef was born. Yosef was born right about the time Yaakov finished his 14 years of service to Lavan. Thus Yaakov was 77 when he arrived in Haran.
According to the above logic, 14 years (77-63) are missing between when Yaakov left and when he arrived. What did Yaakov do for this period? The logical choice for an "ish tam yoshev ohalim", one who dwelt in the tents of Torah as Chazal understand it, would be to learn in yeshiva for this period.
How do we know that the yeshiva was run by Shem and Ever? The assumption here seems to be that some holy tradition, let us call it "Torah", was passed down through the generations prior to Avraham. Noach passed the tradition to his (most righteous?) son Shem. When Shem died, his oldest living descendent was Ever. When Yaakov went to Haran, Ever was still alive (Shem had recently died). If Yaakov wanted to learn this tradition, he would do best to go to Ever, who himself had learned from Shem. Thus, the midrash refers to the yeshiva of Shem and Ever.
If you think the whole idea of 14 missing, unmentioned years does violence to the pshat of Sefer Breishit, there is hope for you. One can think of other explanations of the Yishmael/Nevayot verbosity, in which Yaakov went directly to Haran, and Yishmael died 14 years before Esav's marriage. Here are two possibilities:
- Since Yishmael was dead, Nevayot is mentioned because he is the one who married off Machalat. (Yishmael is mentioned to indicate the family relation. Nevayot is an obscure character, and mentioning him would not make the relation clear enough.)
- Yishmael had multiple wives, and verse 28:9 wanted to emphasize that Esav's wife came from the highest-status branch of Yishmael's family. (Verse 21:21 mentions only one wife of Yishmael [an Egyptian woman]. But the point is likely to illustrate Yishmael's distancing from Avraham's family, rather than to provide a complete description of his family life.)