As unbelievable to the viewer that no one catches on to the charade, in-universe the brothers are constantly misdirecting in questions to plastic surgery, which is so normal and casual in this society apparently that no one questions them beyond that.
It implies that the Wilson sisters themselves had plastic surgery, so the Copeland brothers using that alibi is not surprising.
It could also be along the lines of American Psycho, whereas all of them are so incredibly vapid and self-centered that someone they considered a friend can look completely different and people do not notice.