It's not immediately obvious, but as soon as you realize up to WHICH point Maho has sacrified her own happiness for Miho's sake, you discover the poor girl's whole life has likely been a walking Tear Jerker. Then you begin to read more deeper into her lack of expression, and it starts to move into horrific territory.
The bittersweet ending of Little Army may be a Tear Jerker alone but if you combine its aftermath (Miho following Nishizumi-style Sensha-do despite Maho's sacrifice) with Maho not having our viewer's knowledge of the Anime series, which means that Maho spent the time betwen prequel and series believing her sacrifice had been in vain, she becomes a failed Tragic Hero of river-overflowing proportions. Tears which she can't shed if she is to mantain her facade. That leaves us.
The real point at which Maho's plan fails is when Shiho announces her intention to disown Miho for not being a good Nishizumi, meaning that even if Maho is the heiress, Miho is still forced to live by her mother's rules. At that point, she abandons part of her facade, and begins expressing her opinion about Miho in front of their mother; one can only imagine what she's truly feeling.
Hana getting disowned by her mother for doing Sensha-do in the fourth episode. Hana does end up reconciling with her mother in Episode 10, but that doesn't make it less sadder.
In Episode 7, when Saori tells Miho that Mako had an argument with her mother before her parents' death, it becomes apparent why she is so desperate to reach her grandmother at the hospital as soon as possible (and to an extent, why she's afraid about her reaction if she's held back); she's most likely scared she'll part from her grandmother the same way.
The student council trying to strongarm Miho into joining the tankery club is this when it's revealed that the school must win the tournament or face closure. They aren't acting out of callousness or a desire for glory, they are acting on sheer desperation to recruit the only person in the school that has any experience in tankery.
Miho's initial reaction to the aforementioned strongarming is also a little harsh, resembling post-traumatic stress. After being given no chance to turn down the council, she's left with Empty Eyes and sits in a state of what looks almost like shell-shock for the better part of her classes. When the teacher calls on her for a question, she hardly reacts, much less snaps out of it; when she's excused to the nurse, she can hardly even maintain her balance when she stands up.
Mangas
In Chapter 6 of the Little Army manga, Miho and Maho's mother arrives, and Miho watches her sister go into full Ice Queen mode (particularly when she confirms Emi's accusation and coldly asks "Was that all you wanted to talk about?"). Given that previous chapters and flashbacks had indicated that Miho greatly admired Maho, and a flashback shows her going into Sensha-do because of Maho, she's devastated as a result.
In a very strange way, this scene actually doubles as Heartwarming. Initially, Shiho is clearly impatient and dismissive and yet, the moment Miho says that her question is important, she stops and is willing to take the time to answer.
Chapter 12 of the main series manga portrays the scene in which Oarai is given three hours to surrender to Pravda, but with a cruel twist in that adaptation. In this version, it's indicated that Miho knows that if she doesn't win here, she will be disinherited, and yet outwardly, she is no less certain of her initial decision to surrender and avoid the possibility of her friends being injured in the skirmish. Yukari, the only one there who knows about the personal stakes for Miho, finds it almost unbearable having to watch Miho make this decision.
Yukari: (thinking) Mi... Miss Nishizumi...! Why!? Just why Miss Nishizumi! Accepted your disinheritance? Are you fine with just that!?
Chapter 15 of the manga adaptation, in which Erika gives Miho a brutal "The Reason You Suck" Speech, and causes her to break down. In the same chapter, it's also revealed that while Miho saved the lives of her teammates, they were ostracized by a large portion of Black Forest and ultimately transferred out of the school, much like Miho did, causing Miho to wonder what, if anything, she accomplished, and believe that in the end, she failed to protect anyone.
The Phase: Erika manga shows how lonely Miho was at Kuromorimine, with most of her schoolmates not thinking much of her. Maho ended up having to be distant from Miho due to the demands of her position, and insisted that Miho call her "Commander" rather than "Onee-chan."
