http://revach.net/article.php?id=3013

Section: Parshas HaShavua   Category: Quick Vort
Parshas Chayei Sara: Rav Yehonoson Eibushitz - Why Does Avrohom's Daughter Suddenly Show Up?
"V'Avrohom Zakein Ba Bayamim VaHashem Beirach Avrohom Bakol; Avrohom was aging and moving on in years and Hashem blessed him with everything." (24:1).  The medrash says that the word "Bakol" alludes to the fact that Avrohom had a daughter.  Why does the Torah tell us this in the Parsha of Yitzchok's shidduch?

Rav Yehonoson Eibushitz answers that Avrohom was now a widower and needed to remarry.  The halacha is that if a father needs to choose between getting married himself or marrying off his son, he should get married first.  However, if the father already was Yotzei his mitzva of Pru U'Rivu, then his son comes first.  We all know that Avrohom had a son, but we didn't know that he had a daughter, and, therefore, he should have remarried before dealing with Yitzchok's Shidduch.  Therefore, the Torah tells us that Avrohom already had a daughter, and his first priority was to call Eliezer to go find a shidduch for his son Yitzchok.