For the Candace+Cassandra TV scene to happen:
First, you have to have heard the rumor from Nick about watching the "Fantasy Adventure Drama." So ask Nick for rumors and he'll eventually tell you: "There's a certain medieval fantasy show that's pretty popular... maybe you should watch it with your whole family." If he tells you: "I'm not sure there's anything else to tell you. You're as up to date on rumors and gossip as I am." Then you've already heard it.
Another thing that is necessary is that you've had the dinner date with Candace (the first time you've had sex).
You also have to have had the the scene with Cassandra where she visits you in your bedroom at night for sex.
And you have to have had the conversation with Candace where she asks you if you're interested in Cassandra.
Once you have all that, make sure Cassandra is home, and watch the "Fantasy Adventure Drama" with Candace. She should ask if you want to invite Cassandra. Just say "Yes".
I hope that helps!
You can get through the vast majority of the game with zero mind control at all. You can cast "Charm" on characters at certain points if you want to, all it does is temporarily boost your influence over the character. You can complete the game without ever casting it.
The only real exception is the incest content with Candace (the MC's mom). For that you have to cast "Corruption" on her a few times. "Corruption" pretty much just acts like a key to further content.
There is no real "mind control" where you have brainless followers who will do whatever you want.
Make sure the Incest Patch is working (should say Incest Patch Enabled at the top right of the main menu screen).
Then text Cassandra, then select "Flirt with her."
The first time you flirt with her (or all of the time if the Incest Patch isn't installed) she will say, "Eww. That's a bit creepy [your name]."
The second time you flirt with her she will say, "Did you text the wrong person? I'm your sister you weirdo."
The third time she will say, "Seriously [your name]? Flirting with your sister?"
The only variables that affect those incidents are:
1. The Incest Patch is installed.
2. That you've had the previous flirt message.
Then flirt with her again and, if you have more than 60 influence with her, she will say, "You keep sending me these flirtatious texts and I'm gonna start thinking you're serious."
This triggers the rest of her content.
The next time you're in your house in the evening you should automatically have a scene where Cassandra catches you masturbating.
If, for some reason, that still isn't working, you can try to fix it with the console. Open it and type the following, hitting enter between each one:
Code:
sisFlirtCount = 3
iSisPoints = 0
sisInf = 60
sisFlirtCount can be anything 3+
sisInf can be anything 60+