Deadline is reporting that
Willa Holland will be playing the part of Oliver Queen's younger sister, Thea Queen.
Thea Queen, Oliver’s younger sister. She was 12 when Oliver and her father disappeared. Now she’s a “celebutante” who acts badly. She loved her big brother with all of her heart, and she’s glad to have him back – but now that she’s older and spreading her wings, she might not be ready for Oliver to be Bad Cop in her family and making sure she behaves. She is the only one who gets away with calling Oliver “Ollie,” and Ollie, in turn, calls her “Speedy.” - The TV Addict
The role of the troubled younger sister should be very comfortable ground for Willa as that was much of her role in Fox's
The O.C.. Although she didn't become a main character in that show until season four, filling the void that was created when her on screen older sister (Mischa Barton) was killed off.
CW's Arrow stars Stephen Amell as Oliver Queen/Green Arrow, Katie Cassidy as Dinah “Laurel” Lance, Susanna Thompson as Moira Queen, and David Ramsey as John Diggle.