I am already a gang member. I cant meet her at 11pm in the basement (its locked, although i have the key and i can only hear her through the door till midnight).
I believe I know what your problem is as I've experienced it before too.
When it's 10pm, the icon of your mother will appear in the living room (if you convinced her to stay) and when you visit, she will be laying on the sofa drunk and you have the option to talk to her. DON'T!. Instead, select the basement and you should have the options to take mother downstairs/or take cassandra (it doesn't update/show the correct area she's in). Take Nicole first, then choose cassandra. I hope this solves it for you.
Edit: Just to add, it seems to be a bug with 10pm onwards. On some days if I wait, I get two 10pms, then it goes back to normal. This is what I assume is happening when you take mom downstairs because it's still 10pm when the event is finished and you can just select cassandra instead.