The maid thing would depend on a number of factors, primarily the sister's situation. Such as if she is broke, and is looking for a job, or if she is going to her big brother for a handout.
The main problem with it, is that the MC has a number of people already living there for free, and it would be kind of messed up to make his sister work, when he has these other girls living there rent free. On the other hand, most of them are having sex with him, and sister would not be at first (I assume).
The best way I see it would be to have the sister demand that he let her work of the rent, and refuse to live there for free (something that could inspire the other freeloaders). Even if you do consider her a part owner (though she was not in the will), he does things around the place (in theory), so she would still be willing to do her part.
In closing, I would also like to go on record and say having a sister who is diligent enough to work for what she wants, and not another diva who is expecting to be waited on hand and foot, having everything handed to her. Personally, I think we have enough of them in the place right now.