When the Torah tells us that Avrohom was "blessed in everything" (Bakol), there is a dispute as to what that means exactly.
1. Avrohom and Sarah had a daughter as well as Yitzchak
2. Yitzchak was Sarah's only child and Avrohom did not have a daughter.
3. Avrohom had a "magic healing gem" with healing properties
3a. He was blessed with all manner of riches.
Option 1 is because a person is considered to have fulfilled "be fruitful an multiply" only when he has both a son and a daughter. The small kaf in Chayei Sarah in the word "U'Livkosah" (and to weep for her) shows and indication of "UL'vitah" (and for her daughter. That is it implies that their daughter died at the same time as Sarah. This explanation states that Avraham could not find anyone for her to marry as the entire world was made of of idol worshippers. Unlike Yitzchak, she would have had to go to her husband's family and would have beeen lost.
The second explanation states that for the reason given above, it was actually a blessing for Avraham not to have a daughter.
When Yitzchak went to Geror, he told everyon that Rivkah was his sister. Avraham by this time was well known and therefore, it would have been well known who his family was and whether or not he had a daughter. For the claim to be believed, the fact of a daughter had to be known.