Its random event, if you have minami at 50 kink points just wait. when it arrives you have 2 choices, first (side with minami OR side with sasha) you have to side with sasha, second (let it go OR punish minami) you have to punish minami. you slap her ass, she likes it, and her kink points are unlocked to 100. For me it was near the end, she was part of home harem already.