 Leigh first appeared on the show during the last two episodes of the third season as Meredith 's half @-@ sister , Lexie Grey . Following Isaiah Washington 's departure , who portrayed Preston Burke , it was reported that show 's executives were planning on adding new cast members , such as Lexie . She was officially upgraded to a series ' regular on July 11 , 2007 , for the fourth season . On casting Chyler Leigh as Lexie , Grey 's Anatomy creator Shonda Rhimes said : " We met with a lot of young actresses , but Chyler stood out — she had a quality that felt right and real to me . It felt like she could be Meredith 's sister , but she had a depth that was very interesting . " 
