And wow, it was a big reveal.
Spencer only found out about her in the series finale. In the episode, Alex kidnaps Spencer and holds her hostage in an attempt to take over her life.
Alex was living peacefully in England without knowing anything about her family in the United States, until one day, when Wren showed up at the bar where she was working.
Wren tells her about her family and introduces her to her half-sister, Charlotte.
Charlotte, of course, was A, torturing the liars back in Pennsylvania. So as far as Alex knew, the liars were just some mean girls, because that's what she heard from Charlotte.
Charlotte leaves Alex in Europe, telling her she just has to torture the girls one more time, as one does. The next time Alex sees Charlotte, she tells Spencer, was at her grave.
Charlotte's death triggered Alex's descent into A.D., as she becomes determined to figure out who killed her.
Although it was a twist, there were several clues hinting that A.D. was Alex.
1. The Obsession With Charlotte's Death
A.D. and the board game only showed up after Charlotte died. A.D. was desperate to figure out exactly how that death happened.
This means that whoever was A.D. actually cared about Charlotte -- there was an emotional connection.
This also means it likely was not somebody who was around the night Charlotte was killed, because then she would've had more clues to figure it out herself.
So, who's somebody far away who cares about Charlotte? Why, her half-sister, of course!
2. That Airport Scene
In order to know that Alex was A.D., you'd have to first have figured out that Alex existed.
The show did, in fact, give some clues towards that. The biggest of those clues was a scene this season at the airport.
Ezra is there and sees Spencer. She's talking to Wren, and looks shocked to see him. She introduces Ezra to Wren and asks that he not tell anyone about the meeting.
And then, nothing else happens with that scene. None of the liars ask her about the meeting, and we never see Wren again (until the finale explanation).
So really, Ezra should've figured out Alex was Spencer's twin, and not Spencer. That's on you, my man.
3. All Those Toby Hookups
Throughout the second half of Season 7, Spencer had sporadic hookups and flirtations with Toby.
And before this season, there was that strange goodbye kiss Spencer shared with Toby.
The weird thing about those hookups was that Spencer never talked to her friends about it. You can bet there'd be chats if it were really happening.
4. "Spencer's" Appearance In The Shed
At the beginning of season seven, Hanna is being tortured in some sort of wooden shed.
As she's in complete despair, Spencer comes to her and gives her comfort, telling her she's going to be OK.
When this happened, we thought it was just a dream. But turns out, it was Alex.
5. The Prevalence Of Twins
I mean. That family's got twins in them genes. Did you really not see another twin reveal coming?
6. Basically, Everyone Else Was Out Of The Running
By the end of the series, pretty much everyone had a reason why they couldn't be A.D.
Either they were too close to the girls, too far from the plot, or they'd already been A at some point.
Really, they had to bring in a twist to make it all happen.