Some things are worth going to jail for. Even for a long time. This is one of them.
Tell her what they're going to do, show her what is likely to happen to her if she allows them to take her there, take her and run--across state lines, hide under and assumed name, abandon all your life baggage--home, job, friends, relationships and just go.
The world is full of cases where people didn't get out of some horrible situation and died because they didn't know when to run or, if they decided to run, they looked back and tried to go back and take their stuff. Empty your bank accounts completely, pack a couple of bags, charge your credit cards up to the limit with non-perishable foods, socks and underwear, ordinary clothes including some a size or two up if she's still growing, long-wearing shoes.
Don't forget a can opener and hot plate. If you can afford it, charge one of those converters that lets you plug in stuff and runs off your car battery. Take a bag of charcoal--parks frequently have charcoal grills.
You won't be able to use your credit cards or your ATM cards again without giving away your location, so fill them up with stuff you will need--including a resume or ten under assumed names with fictional qualifications of stuff you really can do. Only use xeroxed copies of these, on good paper, for job applications. Your computer access will be iffy.
Buy and fill a couple of gas cans to extend your driving range before you have to talk to anyone.
Pick a direction and do not stop until you are out of gas. Cross the Canadian border with her--you'll have to use real ID and say you're visiting friends on vacation. Pick a direction and keep driving. Sleep in the car, go potty in the woods. Sponge bath, wash in the car while your gas holds out. Use your water to wash beside the car. Take bleach and refill your big (at least 5 gal) water container at streams. Wash in stream water, cook and drink stream water you've treated with a few drops of bleach. If you let it sit in air, the chlorine evaporates off and you almost certainly won't get anything from the water. If you do, a doc in a box at your destination can cure it easy.
Do not stop and give *anybody* a chance to see you no matter what. They will track you as far as the border based on your real Driver's License (check--I think you can cross the border on vacation with just a DL still--if it would take even a couple of days to get a visa or clearance or something, stay in the US and see if Mexico lets you cross from the US over on just a DL, but Canada's way closer to you.)
Canada has the advantage that they might believe the kid's in danger if they extradite her and keep her regardless of the treaties. Mexico has the advantage that if you can by hook or by crook scrape up enough money, anybody can be bribed.
Do not, do not, do not ever use an ATM card or credit card ever again. Charge the cards up and fill up in the middle or side of town before you leave, not in the direction you flee.
Don't pick aliases that have your first or last initials--most people do that.
Work at hole-in-the wall off the books places, with illegal immigrants, and work hard as hell. Live among them. People aren't too likely to recognize you, and they have too much to lose if they turn you in. It's probably okay to tell a few illegal co-workers or neighbors a little if they ask too much. Not the name of the school, but that you're getting her away from child abuse. She'll need a job, too. If the illegals have their kids in school, it's probably safe for you to. It means the local authorities maybe are sympathetic to them and don't want higher officials coming in and making trouble.
Keep your mouth shut. Never contact anybody you used to know. Ever again. Not until after she's 21. Not anybody, not ever. Doing that one thing is what gets most people caught. Dear old so and so who would never narc you out WILL. Even if they don't, if they're the type not to narc you out, the authorities or a private detective will be tapping their phones and watching them--or will have turned their friends or neighbors--and will catch you anyway.
I mean it. Everybody has somebody who they think is "safe"--and they get caught over and over again that way. Even if good old Aunt Millie WAS safe--herself.
Resist temptation to break cover.
Don't work at what you do now. Preferably, leverage your skills sideways so you don't work at a job you EVER worked at. If you want out of minimum wage again, learn completely new skills.
Your neice may have to stay out of school and lie about her age. Or not. Canadians don't like homeschooling and it's often not legal there. In any case, drop her out as soon as it's legal or she looks old enough to the legal age for dropping out to lie. Let her study on her own and take the tests later, when she's grown and it's safe to take them under her own name.
You'll quite possibly go to jail for a long time when you get caught--which will hopefully be after she turns 21 and you can try to return to whatever normal life you once had.
The important thing is that you will have saved HER from being traumatized and having nightmares the rest of HER life--or whatever other horrible harms (including death) other kids have suffered in these places, as well as their nightmares for life. She will be able to go back to her real identity at 21 with no legal consequences for ever leaving it.
I said it before. Some things are worth going to jail for. This is one of them.
Screw self interest. She's only a kid, and she has the whole rest of her life ahead of her. Save her. Adults save imperilled children who can't save themselves. The world is a shitty place where often mass-murderers die of old age, comfortably in bed. Women get raped in dark alleys. Innocent people die cold and alone, of torture, under one madman or another, and frequently the Knight in Shining Armor never comes to the rescue.
Every adult's primary responsibility is cushioning children from that basic truth, keeping them in a world where the villain always loses at the end of the story until they're old enough to take up the burden themselves.
Sure, we squeeze what happiness we can out of the deal, but the world is a hard, dangerous, shitty place and we can't ever change that fact. We can find ourselves a little corner of happy and peace now and then if we're lucky, but that happy and peace is a thin layer of nail varnish over the dark truth.
We change it as much as we can, but it's never enough, and never will be. It's just a hell of a lot better than it would be if we gave in and gave up. You can't win, you can't break even, you can't even quit the game--but you can make it a hell of a lot less shitty if you try.
"The only thing necessary for evil to triumph is for good people to do nothing."
You've already probably had a little corner of happy and peace. Now she needs a shot at one. Besides, who knows? You might find a good new life, not be prosecuted, or get off light. If you don't do anything she's DEFINITELY going to get at least parts of herself permanently broken. You're the only knight she's got. If you don't ride to the rescue, no one will--not for her.
Be the adult you obviously are and save the kid.
Julie