The letter kaf in the word “v’livkosa” (to cry for her) is written smaller than the other letters. If that letter is ignored, the word would be read “u’lbita” – that Avraham went to mourn for Sarah “and for her daughter,” suggesting that Avraham and Sarah had a daughter, who also died. The Talmud in Baba Basra (16b) discusses Genesis 24:1, that G-d blessed Avraham “with everything” (“bakol”). Rabbi Yehuda’s opinion there is that this means that Avraham must also have had a daughter. Some say that this girl’s name actually was “Bakol” (“with everything”).