I'm wondering if this odd "relationship" between the brother and the master is a bit of a... Just to avoid the word cope... A way to deal with their loss to some extent. Like they know that their relationship, however fiery it is, was part of that life when the original Miss Sunflower lived (Or at least the one that was "their" miss sunflower) and as such, they cherish it in some odd way.
Although that would make this situation a bit bitter and maybe even a tiny bit toxic for both of them. Which I would never really consider from this story. So far while it did have some negative stuff in it, it never was at such an intense level as this would imply.
This chapter made me modify my hypothesis about their relationship somewhat, in that maybe there's a bit less romantic attraction to sensei on the brother's part and more professional and romantic (in regard to the first Miss Sunflower) rivalry. Sensei goes to great lengths to demonstrate that she doesn't take the brother seriously as an adult, so his relationship with her entails a lot of attempts to prove her wrong. That is, their relationship isn't only about their feelings for the first Miss Sunflower--she appeals to him as a professional role model, and, as the last couple chapters made explicit, he triggers her latent nurturing impulses.
Still, sensei is the only character in the series who is designed as both "sexy cute" and mega-flirty and the brother is the primary adult male, so it's hard not to see his (at least) interest as also possibly romantic.
