I am in the same situation, I am not progressing in any way with Cassandra. She and the mother have more than 100 points.
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.
I'm not sure what could possible cause those not to fire, but if you want to 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+
Those are all the variables that control the start of Cassandra's content (other than the Incest Patch). Hope this helps.