Emily doesn't know Chris is a sleeze ball, he's supposed to be handsome and alluring (as much as the game can make him, anyway) and Emily is both after a recent break up and didn't have sex for months. Given all these it's absolutely not surprising that she'd normally hook up with a guy as a rebound rather than obsess "oh but my little brother might possibly mind this".As much as I normally hate this narrative in this exact case not having an actual specific opt out would be one of the rare cases where NTR content actually cheapens the LI outside of the NTR routes. It basically means that the only thing stopping Emily from NTRing the MC is a magic button from the beginning of the game, like it requires divine intervention for her to not sleep with that sleeze ball.
If acting like a normal human being "cheapens the LI" for you, well. Twice so for those who have their own MC bang random girls at every opportunity.