What is best for daughter?

What's the point of forcing her back home when she's going to be able to move out and be with anyone she wants within the next year? As long as she's not involved in anything risky or illegal where you could end up being legally or financially responsible, and as long as biomom is providing on her own dime, I'd let her stay.

As soon as she is18, she'll take her sassyass back to bio mom. She is rebelling and her bio mom likely feeds that.

Let her go. She'll be back atthe first sign of not getting her own wayall the time...most likely.

The ungrateful little brat!

Empty her room, store her crap in boxes and paint it a great new color for a guestroom. That girl needs a rude awakening.

Legally she needs you to give guardianship to her original mother if she intends to live there as that person has no authority to make medical decisions for her, sign permission forms for school, etc.

It would be easiest for her to stay at home until she is a legal adult.

'We feel she will be safe" That is your answer, as well as making it clear that can she alwasy come back, no questions need to be answered. 

Almost 18, she will take off anyways if that is what she feels like she wants to do. Best to allow her to feel like it was her choice, and leave the door open.

How would we know what is best for a 17 year old we know nothing about and have never met?

No one here has a clue why she doesn't want to stay with you and why she prefers bio mom.

You are responsible for her until she's 18.  Calling the police is probably a bad move - at least a bad first move.

Have you talked to bio mom?   I would have told bio mom years ago that if she wasn't going to comply with your basic requirements such as sending your kid home when it's time, you will stop the visits.

My guess is that you're a lenient and ineffective parent and now daughter and bio-mom are walking all over you.    Your opportunity to set boundaries has probably long since passed.    She's an adult "soon" so this is hardly the time to start treating her like she's a little kid under your thumb.

Once she turns 18, you won't be able to do anything about her life choices. It is your parental right to force her to return until she turns 18, but if you do so, you'll likely alienate yourself from her.