Amy Acker to Play Derek Shepherd's Fourth Sister on 'Grey's Anatomy'

We’re about to meet another Shepherd sister on Grey’s Anatomy! The show has cast actress Amy Acker as Derek Shepherd (Patrick Dempsey)’s mysterious fourth sister Kathleen, E! News reports. We’ve previously met Liz (Neve Campbell) and Nancy (Embeth Davitz), and of course, Amelia Shepherd (Caterina Scorsone). Kathleen is also a doctor, just like her siblings,