Sforno on Genesis 12:19/ספורנו על בראשית י״ב:י״ט

1?למה אמרת אחותי היא, even after she had already been brought to my palace?

2ואקח אותה לי לאשה, so that I took her to be my wife, not my concubine. I though that seeing that she is your sister you would surely approve of this union!