Sforno on Genesis 42:12/ספורנו על בראשית מ״ב:י״ב

1לא. It is simply not true that you are brothers.

2כי ערות הארץ באתם לראות, you have agreed among your selves to claim to be brothers so that this would provide you with a pretext to act as spies in the land and to find weak spots in our security system.