Omg I just went back to reread some chapters to try to catch more hints, and the scene of the grave in ch 8 showing Konno Yuki's gravestone, guess which family's gravestone (with the kanji name) is shown next to it? Miura family ancestral grave. The same family name of the art teacher Miura. Of course, Miura isn't that uncommon of a surname in Japan, so this might not be the same Miura, but I don't think it's a coincidence the mangaka chose to make sure the kanji of this neighbouring gravestone is clearly shown while the rest of the gravestones aren't even shown with any scribbles of kanji
edit: i went to backread some previous comments on past chapters' threads and learnt that some have speculated that the grandma was from Miura as the wife of the late art teacher. I find that hard to believe because (other than the fact that she's going by Konno, not Miura) it was shown that the grandma had strong enough feelings against Yuki to oppose having her entombed with the Konno family ancestral grave (even mentioning that she wanted Yuki 'alone' and away from the family grave), but Yuki's gravestone was depicted neighbouring that of Miura family gravestone. If the grandma was from Miura and hated Yuki that much, she wouldn't have had Yuki's gravestone just right next door but probably be totally in a different cemetery altogether.
I wonder though if she actually knew of Yuki's crime in full detail and deliberately have her entombed right next to Miura family gravestone...
further edit: Re-reading again, as well as learning some spoilers from TL's reply to me in ch. 12 comments, what the grandma actually said was that when Takeshi died, she "chose a different cemetery" instead, implying maybe it's Takeshi that's entombed separately from Yuki (and potentially Miura family grave, if indeed the Konnos were actually Miuras). So there's still a chance for the Konno = Miura theory.