וַֽיִּשְׁאֲל֞וּ אַנְשֵׁ֤י הַמָּקוֹם֙ לְאִשְׁתּ֔וֹ וַיֹּ֖אמֶר אֲחֹ֣תִי הִ֑וא כִּ֤י יָרֵא֙ לֵאמֹ֣ר אִשְׁתִּ֔י פֶּן־יַֽהַרְגֻ֜נִי אַנְשֵׁ֤י הַמָּקוֹם֙ עַל־רִבְקָ֔ה כִּֽי־טוֹבַ֥ת מַרְאֶ֖ה הִֽיא׃
那地方的人問起他的妻子，他就說：「她是我的妹妹。」因為他怕說「她是我的妻子」，心想：「恐怕那地方的人為利百加的緣故殺我，因為她容貌美麗。」
Rashi
לאשתו means CONCERNING HIS WIFE, just as (20:13) “Say about me (לי), He is my brother”.
Ramban
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.”
Radak
וישאלו...לאשתו, an inquiry regarding his marital status, especially the woman in whose company he had been seen. We find a parallel construction in 20,13. There are numerous such constructions, and I have dealt wit them in my commentary on 12,12.