To say nothing of his looks.
Btw I hadn't noticed before how Mitsuki got her whole fashion sense from her uncle, down to the nails painted black.
I'd say, it's the other way around - the uncle copycatting Mitsuki as her approach with black nails brought cute girls into the shop, lol. I mean, he never wore it black before this chapter.
In reality though, not gonna be surprised if this is but the author's way of "bugfixing" that nails painting are a girls' only thing, as in, hey look, actual guys of the culture do it too, so it's not like Aya fell for feminine traits of her Onii-san. Not that it addresses the elephant in the room, which is the voice of a 17-year-old. But at this point, I don't think I want to care about any of that as for me personally the momentum of the story is somehow lost.