Pretty Little Liars: Is Spencer's Sister Melissa Really "A"? (VIDEO)

Melissa Hastings is looking guiltier by the minute.

On the February 20 episode of Pretty Little Liars, the girls of Rosewood were shocked to find a video of Spencer's older sister Melissa (Torrey DeVitto) consorting with the three people they believed murdered Alison DiLaurentis.

In a sneak peek from Monday's all-new episode of the ABC Family hit, Hanna Marin (Ashley Benson) accuses Melissa of being the treacherous "A."

"You guys, my sister is not 'A,'" Spencer (Troian Bellisario) tells Hanna, Aria Montgomery (Lucy Hale) and Emily Fields (Shay Mitchell). "I watched her fall apart when we found Ian's body. She had no idea that he was dead! She wouldn't just text herself from her dead husband's phone and lose herself like that."

Unconvinced and scared for their safety, Hanna suggests turning the incriminating video over to the police. "We have video of four people who could have killed Alison," she reasons. "They were in her bedroom the night she died."

Spencer, who doesn't know what to believe, then begs her friends for more time to asses the situation before making a rash decision.

"I know that I'm asking a lot, but I think that I might have a way to find out more," she says. "Can you guys just give me a chance to get into this?"

Pretty Little Liars airs Monday at 8 p.m. EST on ABC Family.