If:
1) She hasn't been trouble
2) She doesn't play-out her mom and dad issues with you
3) You are ok with her staying (this is a huge one, think carefully)
Then:
1) Explain to her that she will become a household member, not a "guest". Guests have things brought to them and are cooked and cleaned for -- household members hold-up their end of the bargan.
2) Explain to her that you are not her father or mother. The first time she starts treating you like one or the other, she'll be looking for a place to stay.
3) Even if things go well, you want to sit down with her and your daughter on the last weekend of every month and talk about what's working and what's not working. She's a month-to-month guest and should never take her residency there for granted.
4) After 90 days, pick a modest number for rent and utilities she'll need to pay. If you are feeling generous, make it some small, token amount. Just as long as she has to find a job of some sort, you should be ok.