וַֽיִּשְׁאֲל֞וּ אַנְשֵׁ֤י הַמָּקוֹם֙ לְאִשְׁתּ֔וֹ וַיֹּ֖אמֶר אֲחֹ֣תִי הִ֑וא כִּ֤י יָרֵא֙ לֵאמֹ֣ר אִשְׁתִּ֔י פֶּן־יַֽהַרְגֻ֜נִי אַנְשֵׁ֤י הַמָּקוֹם֙ עַל־רִבְקָ֔ה כִּֽי־טוֹבַ֥ת מַרְאֶ֖ה הִֽיא׃
Und die Leute des Ortes befragten ihn über seine Frau und er sprach: Sie ist meine Schwester; denn er fürchtete: Wenn ich sage, sie ist meine Frau, so könnten mich die Leute dieses Ortes wegen Rebekka umbringen, weil sie so schön ist.
Rashi on Genesis
לאשתו means CONCERNING HIS WIFE, just as (20:13) “Say about me (לי), He is my brother”.
Ramban on Genesis
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.”
Haamek Davar on Genesis
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.