How could Avimelech not know that Yitzchak didn't have a sister?

Question

Breishis 26:6 says :

6. And Isaac dwelt in Gerar.
7. 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?

Answer 1

Toldos 26:7 shows that it was not Avimelech but the residents of the place who asked him as soon as he arrived. He then, as he later explained to Avimelech, suspected that they would behave in the same way as the Egyptians would have behaved with Sarah. In fact as Rav Hirsch says

That Isaac's precaution was not without cause is proved by the fact that, once the true relationship between Rebecca and Isaac became known, Avimelech found it necessary to protect them by a decree of the death penalty for any assault.

Indeed we see that they started feeling less threatened as time went on that they became less cautious in their behavior and Avimelech realized that they were man and wife.

We should note that this was after the death of Avraham so that Eisav and Yaakov were old enough to masquerade as members of the extended clan rather than their sons.

Since this was a matter of the general populace rather than Avimelech, they would have a vague idea of who Yitzchak was, but not a full realization of the family relations. Note that they had filled in the wells that Avraham had dug and apparently tried to bury the history involving Avraham and his teachings.