It’s equally unreasonable to insist that everyone own a desktop/laptop style computer.
Sister B’s actual needs for e-communication may well be satisfied to her satisfaction by her mobile-only approach. Whether that make her Bro the OP happy is another thing. But ultimately that’s Bro’s problem, not Sis’s
Having said that I do share the OP’s frustration. There’s a sensation of casting pearls before swine when you write a detailed essay and get back a garbled half-thought. Part of which is due to the keyboard limitations discussed above.
But a lot more of which is driven by how we use our phones: as boredom abatement devices. The goal is to “get rid of” the emails. We do this while doing something else like watching TV, eating, etc. We’re barely paying attention to the words in the screen before we click out a quick response or better yet just push [Delete]. It’s that non-engagement with the real content of the real communicative intent of the sender that precludes a good response.
I email my volunteer group a few times a day soliciting input on various topics. Most folks read & respond on a PC. I get thoughtful complete sentences from them modulo their individual aptitude for writing which varies greatly.
One person dictates their response into a voice-to-text app. Sometimes on his tablet, other times on his phone.
It might be two or 4 paragraphs long, but it’s a stream of consciousness mess. So the ideas upthread that the keyboard itself is the problem and text-to-speech is the solution may not be as true as we all wish it would be. Even overlooking the inevitable word choice errors the TTS process makes, dictation is a linear process that, absent a real keyboard, is practically uneditable. And absent the give-a-shitness inherent in organized deliberate application of your communication skills, just results in what I get: Dictated stream of consciousness, followed by [Send], [Delete], [Show next email]. Lather rinse repeat. All while watching TV or riding in the car or …
See also this recent thread on essentially the same problem: