Now I can see the logic in that, especially if the sister learned the art of shibari whilst in Japan but I earnestly hope this is not going to be the case. Coercion needs to be in the blackmail routes. (Bring back the 'full Bennett' I say). This family of immature geniuses & their censorious, overly pious mother doesn't, IMO, generate an atmosphere of eroticism, just weirdness. Yes, they can stir the imagination but again IMO, not in a good way.
Considerations of appropriateness aside & thinking of realism, there was no way that Sophia, noting her previous character, would have indulged in 'hands-on' sex education in the way she did. There was no logic to it from a teacher with a decade's experience, ensconced in a boy's bedroom with three tenagers, whatever their actual age.
Overall I wish the two side-jobs could be written out, saving time & controversy. Let Sophia model garments for Liam's company, focussing on fashionable clothes for the over-35 age group. That really does fire my imagination, the outfits, the glamour, the photo-shoots, indoors & out, chance encounters, flirtations, affairs? It's so good.....but instead we've got two old blokes recuperating & a family from a psychological drama.