Small text on how I got the stepsis (WSIS) bonus scene (sorry for any English issues or confusing text):
-make sure you have the Sensuality trait (so girls gain lust from normal physical contact, such as hugs)
-be part of the student council and give her swimming lessons (having the sensuality trait will increase her lust level)
-hug WSIS a lot through the day (high intimacy will block the hug timeout, so u can do it many times)
-complimenting and chatting also rises the lust levels
-after lots of hugging and giving her swimming lessons (which will already give you a high lust level), invite her for a late-night snack. The accidental breast-grabbing scene will increase her lust and reach over 50%, unlocking the scene.