I'm the same, I think it's a bug, because it shows that Susan is bathing, but when you go to the kitchen, she's also there with Kevin. My theory is that it's a bug where Kevin should be alone, but it never happens.
inside the game if you open the console with Shift + O on Windows and type renpy.jump("susan_threesome_intro"), you can forcefully start the threesome event. However, I'm not sure if this could cause any bugs—it's essentially a brute-force way to trigger the scene.