I think putting up with this for 2 years means the OP has shown more than enough patience in expecting her DSD to become a contributing, respectful member of the household.
I also think that the girl's father, OP's DH, has also had those 2 years to actually parent his child and tell her what is expected and follow through with consequences.
OP has had enough, and I don't blame her. I think DSD needs a harsh life lesson that her behaviour and failure to do anything that is expected of her is just not acceptable. She cannot continue to live with you and take the piss, and that's exactly what she's doing.
DSD's mother has told her the same thing, which is why she's still with you.
All her parents and step parents need to stand firm on this. Help her find a room, a bed sit, perhaps pay the first month's rent, but she needs to realize she's a grown up now who is not in education or training, her choice, so she needs to find a job and support herself.
Good luck, OP. I don't think you're being unreasonable. If she shows genuine movement towards making an effort at becoming respectful, cleaning up after herself, and looking for and going to work, then maybe extend the deadline. But I do think living on her own and soon will be a wake up call that she really, really needs.