Your theory would have to be that she was incapacitated / diminished capacity. If the facts support this, you may have a case, but it will largely depend on the facts. Unfortunately, your daughter is over 18 and it sounds like she just decided to check out. The neighbors will likely say that they just gave her a ride - that she left on her own.
I think your neighbors are jerks, but your niece is an adult and she wasn't committed, just seeking voluntary inpatient treatment. I have a hard time believing picking someone up from a rehab clinic could create a legal basis for suing them. You might be able to seek a refund from the rehab clinic for the services they ultimately didn't have to render or sue your niece because I'm assuming you conditioned the payment on her agreeing to complete rehab.
Your best bet is to seek some money back from the rehabilitation facility. Your daughter has the right to stay, or go. She is not a prisoner. It would be false imprisonment on the part of the rehabilitation facility to keep her against her will. There is an old saying that you can lead a horse to water, but you can't make it drink. This is ironically true with alcoholism, and drug addicts. As much as you would like to help your daughter, and I admire your efforts, your daughter alone has to make the commitment for it to work. She gets to choose. You can't do it for her. She has to do it for herself. Her leaving indicates a lack of commitment, contact the rehab facility to see if you can get any money back for the unused services. Al-anon is a valuable service for people effected by other loved ones addiction. They can be very helpful.