I wasn’t planning on posting this theory, because the
evidence is shaky, but @pi-romantic said they were interested to hear my thoughts, so
I’ll share! Besides, even if there’s not much to stand on, my family fluff
loving brain wants this to be true ;P

Before I begin, I want to take a moment to talk about the
change in dynamics between Tj and both Becky and Wordgirl. I recently noted that
the relationship between Becky and Tj was very different in earlier seasons
than later seasons. In the beginning, they had a very stereotypical tv sibling
relationship where they were always annoyed with each other, while later
episodes they had a more realistic relationship: they get on each other’s
nerves, but they still care for each other. And changes weren’t only between Tj
and Becky, but Tj and Wordgirl as well. Earlier seasons it was pretty clear he
had a crush on her, while later in the series, I feel it grew to be more of a
fanboy dynamic, where he idolized her, but maybe didn’t necessarily have a
crush on her. I bring all this up to explain why Tj’s reaction to finding out
his sister is Wordgirl didn’t result in the same reaction as that of in the
episode “Two Brains Forgets.” It’s because the dynamic between him and both her
egos are very different at this point in the series that it was at that point.

Now that I’ve got that out of the way, when did Tj figure
this out? I believe it hit him and/or he started to get suspicious in “All that
Chazz”

“All that Chazz” focuses on Tj idolizing a new kid, because
he has a crush on him, Chazz. At the beginning of the episode, Tj tells Becky
that he thinks Wordgirl is the coolest person ever, however, Tj starts
pretending he doesn’t believe this when Chazz states that Wordgirl is uncool.
After a while, Wordgirl, getting frustrated, goes to Tj and says, “Tj, may i
remind you that just this morning you told me you thought Wordgirl was the
coolest person?!” and Tj says, “Yeah but…wait, I didn’t tell you, I told my
sister Becky….”

This is far from the first time she’s slipped up like this,
nor is it the first time she’s slipped up in front of her family, but to me,
this one was different. Usually, it’s something like her accidentally saying
“mom” or “dad” or saying “our parents” instead of “your parents” etc. Those can
be fixed easily. She usually just pretends she said something else and acts
like it never happened and no one’s the wiser. After all, they’re minor
mistakes and they could have heard her wrong or she could have had a
tongue-tied moment and got her words mixed up.

But this is a very specific error that’s hard to recover
from. She did recover from it, saying “Right! Yes, I knew that..uh, but you
see, um, she told me what you said. So there! Ha, yes, makes sense!” and
personally I think this is one of her best excuses for having known something
she shouldn’t (although her delivery was terrible). But here’s the thing:
there’s no taking back what she said. It was clear as day, and the start and
ends of her recovery were very awkward. That doesn’t exactly make the comment
seem any less suspicious, and it appears Tj thinks so too, as after she
answers, he has a very perplexed look on his face. I think at this moment it’s
starting to click.

Within the episode, there’s not much else to prove he has any
clue, but watching the rest of the episode with this idea in mind admittedly
makes it a little more interesting.

I did, however, find a moment from another episode after “All
that Chazz” that supports this theory. In the episode “Granny’s Corner,” Granny
May gets on tv to tell people things like “Go visit the bank tomorrow for cake
and dolphins!” While the Botsford family is questioning Granny May’s actions, Tim
says “In my opinion, trying to deceive people on purpose is just as bad as not
telling the truth.” Later, after stopping Granny May’s scheme, Wordgirl recites
this exact quote to the news reporters. She then zips home just in time to
witness Tj and Tim saying, “it’s like she was here!” Tj then dramatically says
“I know what’s going on here!” He takes a suspenseful pause, and then casually
says “super hearing!” This would normally be just a fun joke…except Tj
specifically looks at Becky when he says this, with a smile that to me looks
caring and like he’s covering for her. And while not necessarily evidence
itself, I’d also like to point out that making Becky panic by saying “I know
what’s going on here!” in the most dramatic way possible, only to casually
brush it aside is a very sibling thing to do and totally something Tj would do.

Now this isn’t evidence, but I’d like to imagine that in
“Emergency Plan #999,” Tj was studying all Wordgirl’s battle moves to help her because he was maybe thinking “I’m not only Wordgirl’s number one fan, but I’m
her brother. Shouldn’t I be doing something to help her?”

Also if we assume Tj does know in this episode, it makes the
following joke sssssoooooo much funnier:

Tj: Worddgirl, what’s a stunt again?

Wordgirl: a stunt is an amazing or impressive feat. It’s
something tricky you do to impress people

Tj: my know-it-all sister keeps using that word

Maybe I’ll find more evidence later, but for now, this is
all I got.

If Tj did figure it out, would you have been interested in
that plot development? Also, if he knew and was hiding that fact from Becky,
there probably would have eventually been an episode that revolved around Becky
finding out Tj knew, how do you think that episode would have gone?