There's an event with her and Sasha, it's a one time event, your sister spills milk on Sasha's guitar and you can either take her side or Sasha's, to get it to 100 you need to choose Sasha's side, which will unlock her to go further, I had the same trouble, and had to restart my whole game
Viewing post in Love & Sex : Second Base comments
All the details about this specific Minami's event can be found just there :
https://wiki.andrealphusgames.com/wiki/Minami#There_have_to_be_consequence