Erica McDermott

Erica McDermott is an American actress, best known for her role as Cindy Tar Eklund, the leads characters sister, in David O. Russells film The Fighter .

McDermott was born in Cambridge, Massachusetts. She began her career appearing in Boston stage productions, before she was cast as Mark Wahlbergs characters sister in the David O. Russell film The Fighter . She later appeared in two more Russell films, American Hustle and Joy. In 2015, she also played Benedict Cumberbatchs wife in the crime drama film Black Mass.

Source: Wikipedia