Ah! I see. When I say "my" theory I don't entend to claim ownership of the notion that her brother is the mastermind. Moreso "my" theory relates to the method and reasoning behind his potential actions.
Sorry for the confusion.
Fair enough, and my bad for reading too much into a single word choice.
But it's worth note that, although people have been speculating that her brother (Mitsuha
‡) is Clover almost since he first appeared… for quite a long time, people have also been talking about the method and reasoning behind his potential actions. (Myself included.)
‡ - Georgesmorge's post about "Tooya" (I still dunno who that is) got me curious about her brother's name, so I hunted it down. It's in the section where he becomes her guardian.
A much-abbreviated version of my own old takes: "He's been trying to 'protect' Yotsuha, first from Mommy Dearest (rightly) and then Uichi (probably off the deep end). Time travel is a family power, in a manner that makes me think our mangaka took significant sub-/conscious inspiration from
The Butterfly Effect (2004)
‡‡".
‡‡ - At least wrt it being a family power, and how it works.
If that sounds interesting, and/or you think my earlier MangaDex
‡‡‡ walls of text on the subject could be useful for supporting evidence in the areas where our takes coincide, let me know and I'll try to dig them up.
‡‡‡ - Unfortunately my earliest WoT's and evidence are lost, on a Discord server I left.
And regardless, going back I noticed (possibly re-noticed, my memory isn't the best) something I think you'll also find interesting:
At the same time that Yotsuha gets hospitalized + Mommy Dearest "dies in an accident" + brother becomes her guardian: That's when she thinks Uichi stopped writing.
That is, Uichi's letters were reaching her until then. My guess is that psycho-nii-san started reading them for useful info, then trashing them.