Or HaChaim on Genesis 25:20/אור החיים על בראשית כ״ה:כ׳

1 ויהי יצחק בן ארבעים שנה, When Isaac was forty years old, etc. The reason the Torah reports the age at which Isaac married, something that we were not told concerning Abraham, is in order to to explain why Isaac married so late. The words בקחתו את רבקה, explain that he had to wait that long in order to marry this particular girl. The words: "לו לאשה" "for him as a wife," spell out that only this woman could have fulfilled the role of becoming Isaac's wife. Prior to Rebeccah's birth no בת זוג, suitable potential wife, had existed for Isaac. We have explained in the last פרשה that Rebeccah was very young, having only been born when Isaac lay bound on the altar (at the age of 37) and thus became capable of fathering issue. Under the circumstances no criticism can be levelled at Isaac for not getting married earlier. Rebeccah was only three years old when Isaac married her.

2בת בתואל הארמי…אחות לבן, the daughter of Bethuel from Aram,..sister of Laban. The Torah does not repeat this information in order to remind us who Rebeccah was but in order to defend Rebeccah against possible accusations. Inasmuch as the Torah tells us in the next verse that G'd responded to Isaac's prayer (though both of them prayed simultaneously), someone might have concluded that Rebeccah did not merit that her prayer should be answered by G'd; the Torah explains therefore that the only reason G'd did not respond to her prayer at the time was the fact that her father was the wicked Bethuel. Repeating the information that this Bethuel was an ארמי, is an allusion to the other way of spelling these letters, i.e. רמאי, swindler. The Torah deliberately did not mention the town Bethuel lived in in order to have a chance to allude to the fact that the man was a swindler.

3The reason that the Torah again mentions that Rebeccah was a sister of Laban, something we already knew from the encounter of Eliezer and Laban at the well, is in order explain why such a fine woman gave birth to the wicked Esau. Having both a father who was a swindler and a brother who was a master swindler in her background, the marvel was Rebeccah's innocence, not the fact that one of her sons might turn out to be wicked. Baba Batra 110 already remarks that most sons turn out to have a character resembling that of their mother's brothers. The Torah attests that Rebeccah herself was an absolutely righteous woman when Isaac her husband is described as praying in the presence of his wife. Should you ask why Isaac did not examine Rebeccah's brother's character before he decided to marry her, we have already answered that she was the only woman who was his בת זוג, divinely appointed to become his wife. Had this not been so Isaac would never have married the sister of a wicked person such as Laban.

4The words אחות לבן also allude to the reason that Rebeccah was barren for so many years. According to Bereshit Rabbah 60,13 this was in order that her fertility could not be credited to the blessings bestowed upon her by her brother Laban in 24,60. The Torah therefore alludes to the cause of barrenness before mentioning the fact that she could not conceive. It was because she was Laban's sister that she needed to wait to conceive until her husband's prayer.