Breishis 26:6 says :
- And Isaac dwelt in Gerar.
- And the men of the place asked about his wife, and he said, "She is my sister," because he was afraid to say, "[She is] my wife," [because he said,] "Lest the men of the place kill me because of Rebecca, for she is of comely appearance."
But at that point Avraham and his family were well known in that area. In fact, we know that many important men of that generation, including Avimelech, came to Avraham's weaning party for Yitzchak. It seems like the identities of Yitzchak's immediate family would be common knowledge. Wouldn't the local people know that Yitzchak didn't have a sister?