Yes, there's a scripted event where the younger sister takes a stripping job in another club. Sorry if you don't like this, but that's part of the story. There's no special handler for the situation where she is working for you because that's not supposed to happen. I guess there's still some way you can hire her because you are not the first person to point that out, but she's supposed to be unhireable until that point. If you try to hire her she should say she can't due to some prior commitments or something. If you remember how you did it let me know as that may help me figure out how that's happening.
Anyway, after she takes that job with that club, you can get her to quit (have to wait the 3 months or so) and then you can hire her. That's how the story is supposed to work.
As for the daddy thing, if you are old enough she would be your daughter so that's probably what's happening. Are you sure she is your sister and not a daughter? There was a bug at one point where this wasn't working exactly right so it could be a lingering issue of that but you said you started a new game so I don't think that's it.
You slightly misunderstand. I am frustrated by the younger sister quitting in a scripted event, but what bugs me is that there is a two choice conversation when it happens, after I've hired her, support her leaving or she accuses MC of hypocrisy, and I'm suggesting that this second choice is somewhat irrational, since the problem isn't that she's becoming a stripper, but that she's leaving the job working for her brother. This happens every time, I presume this is something you intended for later in the game, and the verbiage is simply inserted because leave the job is leaving the job. That is where I suggested perhaps a third option, so she can get upset for some other reason, perhaps preventing her from reaching her full potential.
Hiring her was easy, every time I've played 1.09 or 1.10 I've been able to hire the younger sister almost immediately after she showed, by considering for job. The older sister, OTOH, almost always gets nabbed by another club before I get a chance.
MC age 26, pretty sure the 18 year old cannot be a daughter. I've made that age choice each time, each game the younger sister inconsistently refers to MC as daddy.
I will continue to try to get the mother and both sisters employed in the club, but yes, it was a surprise when I considered the mother for a job and she said she was mainly interested in the sex worker position.
I'll also note, while I have your attention, that I often get women who have lewdness as low as -73 who say they will only consider stripper jobs, and to this point it appears that slaves will happily turn down any job offered, perhaps a third of the time, and will also say they will only accept a job they are wildly unqualified for. May be intended as the characters trying to rules lawyer their way out of consequences, but doesn't seem to fit the concept of must accept any reasonable job offer. I've not yet tried to quantify whether it is worth it, compared to my usual procedure of hiring, training and convincing an employee that they love that type of job and everything entailed, based on how often each type leaves.
