פירוש על בראשית 26:7

לאשתו means CONCERNING HIS WIFE, just as (20:13) “Say about me (לי), He is my brother”.

AND HE SAID: SHE IS MY SISTER. They did not ask concerning the children for he would say, “They are my children from another woman.”

He said "she is my sister" and Yitzhak did not request from her that she also say this as Avraham did in a similar situation because Rivkah was exceedingly  submissive to Yitzhak as I wrote earlier at the end of Chapter 24 and for this reason did not deviate from his will in any way.