You have to go to the pool with them and that will trigger an event, then in the afternoon, go take a bath while Sasha is there. She'll ask about her breasts
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Try save-editing the game (or just watching SM/Femdom Porn) with Sasha until she is exactly 75KP. I finally got it to trigger on a save where I spoke to no one except her, and then save-edited her KP to be exactly 75 after the 2nd breast complex event. First time I hit the washroom, event triggered.
She wasn't maxed out on LP either, had triggered the GF event, but I hadn't done the sex slave event yet.