Ah, Genesis 25, the chapter that most clearly demonstrates the non-linear style of storytelling that's used throughout the Bible, and especially in the book of Genesis. (Not to mention a bit of very interesting storytelling in its own right.) In fact, I was reading the commentary in Robert Alter's translation of Genesis, and he notes at the end of chapter 24 that some scholars believe that Abraham may have died before Isaac was married, and that the actual death of Abraham may have occurred while Eliezer was off looking for Isaac's bride.
Anyway, Abraham takes another "wife" after Sarah dies, named Keturah. I put "wife" in quotes because, as the SAB points out, it's not really quite clear whether she's truly considered a wife. A verse from 1Chronicles is quoted, but really, you only need to scan down to verse six, where one can easily assume (as I do) that the term "concubines" refers to Keturah and Hagar. So it might be worth asking what Keturah's status is to some people, although personally I'd suspect that the line separating "wife" and "concubine" is not exactly well-defined. A simple answer might be to point out that, as in many languages, the Hebrew word for "wife" and "woman" is the same. In verse one here, as well as back in 16:3 with Hagar, this could have been translated simply as "woman" and made sense, but perhaps the KJV translators (along with most others) didn't like the idea of Abraham having sex with a woman who was not his proper wife. A more complicated answer might be to consider that the writers of the parts of the Bible that mention this woman might have particular points to make at different times, and so used terms that would accentuate their points. Although Keturah was indeed a "wife" by any normal standard, when we drop down a few verses and the author decides to call her a "concubine", he's trying to make a clear distinction between Sarah and the other two women in Abraham's life. Particularly the relation they have to the promises of God and the land of Canaan.
So Abraham has at least six more children after Sarah is gone, and each of them is sent away from the place where Abraham lives, so that they won't be in the way of Isaac growing up and being prosperous. While I covered most of the objections to this story back in chapter 13, I might as well hit on them just lightly again. Abraham is able to have more children easily because the infertility problem was Sarah's. As far as I'm aware, in fact, while women have menopause, men never stop being fertile, although they certainly may have other problems that would impede them from conceiving. In a sense, there is a viewpoint theologically that nobody has any children ever without God's help, but that's an extreme viewpoint. These other children may not be considered legitimate children if indeed Keturah was a "concubine", I'm not sure. However the point I said back in chapter 13 was that every time Isaac is referred to as Abraham's "only" son, it refers to a time frame before he has any children with Keturah, so the existence of these children has no bearing on the question of how many children Abraham had. Oh, and furthermore on the subject of time frame, I don't know anyone who considers it polygamy to marry another wife after your first wife dies.
Abraham dies at 175, an impressive age, but certainly not impossible. Ishmael comes back from wherever he's living to help Isaac bury their father, so apparently they're getting along okay, perhaps in a large part due to them not having to live together. A short synopsis is given of Ishmael's life and children, who become a powerful nation in their own right as Israel will in the future. Ishmael also lives to an impressive age.
Rebekah turns out to be barren, which the SAB labels as absurd, but doesn't explain why. Perhaps it's the fact that she seems to be in a long line of infertile wives? I don't know what the SAB's getting at, but as I think I said before (and I don't remember where it was if I did) I think that God is putting some odd control on the biological clocks of the matriarchs to make the timing just right for the period the nation of Israel spends in Egypt. Who knows? Maybe infertility was just a problem in that time and place?
Rebekah does eventually get pregnant though, and actually, her infertility is an odd footnote, because it's just for one verse of the story, and we don't know how long in time it was. She only gives birth once, though, and it's twins. More interesting names here; the older one is Esau or "hairy", while Jacob is "heel grasper". Imagine your child being born and you notice that she has a funny nose, so you say to your spouse, "Hey Honey, let's name her 'Schnozia', don't you think?"
Obviously not identical twins, Esau is a big burly hairy man who loves hunting, while Jacob is a bit of a momma's boy (literally) who likes to hang out at home and cook. Their parents play favorites, and apparently, so does God. Despite the note on the SAB, the Bible does not in any place that I am aware of say that Rebekah hated Esau, she just liked Jacob better. In part, it may have been the prophecy she was given about them. Throughout the story of these boys' lives, Isaac keeps trying to make Esau the better one, but God and their mother keep trying to manipulate things to put Jacob in front, because despite not being as manly as his father might like him to be, Jacob is the more morally upright of the two (which says a lot, since he's no angel). We get the first hint of it in the end of the chapter when he sells his birthright for a bowl of stew. Not exactly the shining example.