Pretty Little Liarsdropped a serious bomb in their season finale when it was revealed that Jessica DiLaurentis (Andrea Parker) has an identical twin sister, Mary. Not only that, but Mary is the biological mother to Charlotte (Vanessa Ray), and she is determined to avenge her daughter's death.

Of course, Mary also won't waste any opportunity to stick it to her sister, who she feels got the life Mary deserved. "For Mary, it's about Jessica and Mary's loyalty to Charlotte. What went down between Mary and Jessica in regards to the children will certainly be a fun episode to play," Parker explains to TVGuide.com. "I assume there's a lot of competition and jealousy between the sisters."

Pretty Little Liars' twin twist is a sucker punch for Alison

Show creator I. Marlene King told The Hollywood Reporter that next season will be "the beginning of the end" as the "The liars will make the biggest mistakes of their lives in Season 7 and Uber-A will use it to fuel A's game," she says.

While Jessica DiLaurentis was far from a girl scout, it was clear that she was always trying to do the right thing. Parker explains that the opposite is true for Mary, who is definitely the darker of the two sisters. "Jessica's heart was very much trying to do the right thing --even though she did make some grave mistakes and bad choices and probably drank a little too much," Parker says. "[Jessica] was always trying to do the right thing. Mary, perhaps not so much."

VIDEO: Lucy Hale promises Pretty Little Liars will go out with a bang