Onee-san most likely. I'm guessing one just as talented or more that bullies and/or makes fun of the art Misumi makes. And theres a sense of inferiority or something between them etc etc
Really interesting story. While it's hard to know where they'll take the age gap, the level of writing here is high enough that I don't think I care either way. Thanks for translating!
Kurokawa is so sexy it's crazy. Like I have an interest in softly squeezing and kissing her boobs, especially when she's in her tanktop towards the end of the chapter.