 Lexie Grey is the daughter of Thatcher Grey ( Jeff Perry ) and his second wife , Susan Grey ( Mare Winningham ) . She grew up with her younger sister , Molly Grey @-@ Thompson ( Mandy Siegfried ) and through @-@ out her childhood was kept unaware that she also had an older half @-@ sister , Meredith Grey ( Ellen Pompeo ) , born to Thatcher and his first wife , Ellis Grey ( Kate Burton ) . Lexie is first mentioned when Molly is admitted to Seattle Grace Hospital , with Susan informing Meredith that Lexie is a student at Harvard Medical School . At some subsequent point , Lexie learns of Meredith 's existence . She graduates from medical school and is accepted to take her surgical internship at Mass General hospital , but following her mother 's sudden death from complications stemming from the hiccups , Lexie instead opts to move back to Seattle to be closer to her father , taking up an internship at Seattle Grace Hospital , one year behind Meredith who is due to begin her residency . 
