I feel like playing the mystery buff here and figuring out who the sister can and can't be before it's revealed in the recent Kono Naka ni Hitori, Imouto ga Iru! summer anime series.
What a hilarious and very exciting episode  this was! It's always fun when girls mistake another girl for being a guy and then they think the guy they like is gay. So in the Mei-less episode, an unfortunate misunderstanding causes our poor Shougo to undergo quite a dilemma (for any man, really). But it's all for the sake of comedy, no matter how unrealistic it is, since the last half of the episode is quite heavy and revealing (in more than one way)!
Once again let's begin with the pre-OP, where the cause-and-effect and hilarity ensue. Mister X, who we all know is a girl but not known to everyone else, came up with a brilliant scheme to prevent Shougo's sister from marrying him--for Mister X to marry Shougo first! After learning the secrets of where babies come from, Mister X pushes Shougo onto the bed and proceeds to overtake him, just as these two walk in:
Still thinking Mister X is a guy classmate, they reach the conclusion that Shougo is gay, and thus for the time being, try to "cure" him in very rediculous ways.
But first let's have a conversation with the "imouto" character first, just to switch things up from the first two episodes. As Shougo finishes his shower, he realizes he has a missed call, and when he goes to check, his phone rings--it's his sister. Throughout the awkward chitchat the "imouto" character's bathroom is revealed, so it is very important to keep these images in mind:
Wall decor and lighting. And most importantly...
Furniture/appliance features. Now back to the story.
After a quite fascinating dream, Shougo wakes up to quite a startle. Konoe and Kannagi are in his apartment, acting like his sisters. It is revealed that they are trying "to guide" him back to liking women. But why act like sisters, you ask? Because they happened to have found Mei's flyer that he took for the Little Sister Cafe.
Honestly, I think Mei's only significance is that flyer, which causes the two girls to act like sisters, which makes Shougo (and us) think really hard on which (of the two) could it be. But it's only the third episode (or so you may think).
Carrying on with the day, during lunch they feed him supposed male aphrodisiacs, including this "Hard as a Boss" beverage. I can only assume Viagra in drink form.
Now comes the rediculous part. They go home with Shougo. Then the two girls proceed to yell out erotic things while taking a shower together, just to seduce Shougo. Afterwards they cuddle up next to him while they sleep. And now the most unrealistic part of this hilarious set of circumstances.
They go topless. It would be nice, if he wasn't sure one of them was his sister. He comes up with a plan: if the "imouto" character calls his phone, then the two naked girls grasping each of his arms are not his sister, then he can make a move! But...
Ok, now let's get back to reality and seriousness. The next morning at school, Shougo atempts to flee from the two girls, who eventually lose sight of him. After some menial arguments, they decide to split up to look for him, and it is revealed that they both know that they and each other like him, so they've been trying to get with him this entire time (most likely pretending to think he's gay). So play fair.
Shougo runs into Mana and Rinka, and has a nice chat--nothing too note worthy. Right as school ends, Shougo bumps into Kannagi and they go off to the store because Kannagi wants to buy a swimsuit (I noticed most tsunderes wear one-pieces). Upon leaving the store, it is raining like it's monsoon season, and they both end up drastically wet, so Kannagi brings Shougo to her place to get dried off and showered.
As he walks into the bathroom, he notices the little red "Crabzeus" shampoo bottle on the bathtub.
Knowing Crabzeus is a character from one of his favorite childhood shows, and that his sister gifted him an action figure from that show just a few days ago, he felt compelled to confront Kannagi on this matter that has been eating him up inside.
Simply asking "Who do you want to marry?" should do the trick. But as an awkward question to ask a girl who likes you, she's bound to say yes. And she does. With that, Shougo concludes that Kannagi is his sister, and upon mentioning that he cannot be with her because of that, she simply says "leave".
I swear I've seen this before.
Oh, now I'm positive. Looks like we have a winner, and at episode 3 no less. There could be some undermining circumastances at play here, but it looks to me that Kannagi is the sister!
It almost feels unfair for the mystery to end so quickly, and perhaps it hasn't. But at this point this is what I can conclude:
So if there is still more to the mystery than this, I'll be sure to put up part 4, otherwise, this was fun, and I'm glad to finally see the identity of the sister. Perhaps now the rest of the show can build on a possible future with the other girls, while keeping Kannagi involved too. Thanks for reading!