If one sees sin, is it wajib to express dislike/dissatisfaction even if it wont have an effect?

If you know it won’t have any effect then you don’t have to show dislike. Yes, if not showing dislike will encourage the sin or make it seem like you are ok with it, then you must show dislike. Also, if the ones committing the sin are your immediate family you should also express dislike even if it won’t have an effect.