<P> 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 . '' </P>
when does lexie come in grey 's anatomy
the last two episodes of the third season