You have to be submissive male with your sister having a dominant personality.
Have you tried interacting with her while you're in a submissive state, a bunch of times?
Nothing. The game relies on the Serums to progress a character. Mom and Sister can be forced to have sex with you but after awhile they will never say anything to you and will avoid you. The people in the company will also more or less be the same. You basically cannot unlock 90% of the sex scenes without dosing characters. Not using Serums will basically make the game boring.
They will? How many times does that take? I always found it strange that after forcing one of them they seem perfectly fine the next day.
I am not sure but force yourself on them the first week of the game and you should start getting dialogue that just says "...". They also refuse to be alone with you, often seeking the mom or sister respectively to avoid being with you.
Are you talking about the Growth and Shrink stuff? If so it tells you at the bottom that "Your penis can no longer be grown or shrunk" or something to that effect.
Its when Johny and not the MC uses the potions on everyone to corrupt them to his liking.
You get this when you attempt to dose John while being more submissive than dominant. The easiest way is to blow him a couple of times in the bathroom early on, and then try to dose him.
