I can't believe I never thought of this. Alison might not have a twin sister, but Aria HAS A TWIN SISTER. THINK ABOUT IT. Aria is GREAT at lying, right? So, it would be easy for her to lie about her family only including herself, Mike, Ella, and Byron. When they were younger, Aria's twin could have convinced Aria to impersonate her to humiliate Aria, just like how in the books Alison convinced Courtney to impersonate her. This would coincide how easy it was for Aria to impersonate Vivian. She was used to impersonating. Aria's twin could have been shipped to a crazy institute and the rest of the family lived out lives in Rosewood.
But, on halloween of 2008, Aria's twin escaped and found out that Aria was popular and it ws because of Alison. She started tormenting Ali and was the one who attacked her in the creepy house. However, soon after, she was caught and shipped to Radley. But while there, she met Wren, but he didn't get on the A-Team yet. Ever since then, she's been plotting against Ali ever since. Wren would always make sure the letters would go through the mail to Ali without the Radley staff knowing. Soon enough, Aria's twin and Wren were getting closer. Wren even allowed her to escape Radley with him a day or two. To add, Aria's twin became obsessed with Alison.
But on the night Alison died, Aria's twin escaped with Wren while he kept getting calls from Melissa. Wren let Aria's twin go out on her own as he trusted her and he finally answered his phone to an angry Melissa. Aria's twin made it to Alison who was angry after her argument with Spencer and confronted her. Alison thought she was Aria, but Aria's twin informed Ali of her being Aria's twin. Alison realized that all of the evidence pointed to Aria's twin, not Aria and was fraked out. The twin didn't react positively and killed Alison. All the while, Emily saw the whole scene. Emily was then knocked out by Wren who carried her back inside the barn and went back to Radley with Aria's twin.
I don't think Aria's twin is Big "A". But there could be reason to it. Aria's twin knew that all 4 Liars together are unstoppable because they have each other's backs and would find out who killed Alison. Even if one of the Liars weren't involved in the mystery, it would throw them off. So, before the pilot, Mona was on the A-Team with Aria's twin. Mona's excuse for being "A" was the rest of the Liars taking Hanna away from her. In actuality, there were two reasons she joined the A-Team. At first, she joined because she wanted to torture Alison and the Liars for Alison's torment on her. Then, when the Liars all were in Rosewood, Aria's twin wanted them seperated and knew that keeping secrets can tear friends apart. But, the plan backfired and brought them closer together, which Mona was angry that Hanna was taken away from her and thought it was the Liars' fault.
- In the pilot and first episode of season 3, "A" was introduced. In both, Aria was shown looking at a mirror, wearing the same dress. The pilot can probably clue that Aria has a twin and she is "A", but Aria doesn't know. The first episode of season 3 can probably clue that Aria's twin is back to her "A"-Games,
- She is the only character who makes the “shh” motion in the opening credits, advertising photos, etc.
- “A” told Aria’s mother about her father’s affair through a letter. It would be much easier for Aria to anonymously tell her mother about her father’s affair compared to telling her face to face (and potentially damaging her relationship with her father).
- Aria enjoys both English and writing. This is only a small detail, but could easily connect her to the letters that “A” often sends. Twins usually share the same interests so maybe her twin is sending the letters.
- In UnmAsked, Aria was the only one that took her mask off. I believe the producers said something about the “unmasking of A”.
- Aria is always referred to as the best liar in the group. She even admitted to being “A” once to Hannah’s mom.
- “Miss Aria You’re A Killer, Not Ezra’s Wife. This could point to my theory that Aria tried to kill her twin when they were younger.
- "A" didn’t start stalking the four girls until right when Aria returned to Iceland, but "A" also stalked Alison before she died. This would go hand-to-hand with my theory.
- Aria seems to be the one that is almost always there comforting the girls, and learning their secrets.
- In the Q&A with "A", "A" is asked who he thinks is the missing link, to which he/she responds “I’m more interested in the strongest liar.” In Season 2 Episode 2 “Father Knows Best”, Aria is pointed out as the best liar by all her friends. My theory points out that Aria may be keeping a secret that she has a twin.
- Jason seemed to think Aria was suspicious due to the many pictures of her the girls found in his shed.
- Aria wasn’t in the doll shop when the other three heard the Alison doll chanting “follow me, end up like me.”
- Aria vandalized her dad’s office, she is clearly capable of angry and irrational decisions. This clue would go hand-to-hand with my theory that Aria tried to kill her twin out of anger.
- In the doll shop, when the little boy is asked who was after Ali, he says someone with ‘long dark hair’ like Aria’s. Right after the boy says this, the old woman that owns the shop immediately tells him to be quiet. The little boy was basically saying that "A" is Aria's twin.
- Aria once said, “sometimes you have to lie to your friends.”
- When talking to Melissa, Aria tells her that her favorite ice cream flavor is toffee. And then later, she makes a profile for her mom on an online dating website and the description is “hot mama loves to tango.” At the end of the episode Mona mentions “toffee tango lipstick.”
- In one of the season 3 promo pics, Aria is wearing handcuffs around her ankles. This could hint that she is most trapped and most wanted by "A". Why? Because "A" is her twin.
- Aria isn’t really hurt by "A". She’s never really been directly in ‘As’ warpath...until the halloween episode. I think "A" wanted Aria gone so then they would go after the other Liars. Maybe Aria's twin wanted to save the BEST for LAST, but someone else didn't agree.
Leave suggestions and what you think about my theory. I know it's far-fetched and crazy, but maybe...just maybe it's possible.