Dear Concerned4stepkids,
I don't know where you live so I don't know the laws in your area.
I agree you should take your stepdaughter to the police station and explain the situation.
Your house is your home and its your rules, she has an opportunity to turn her life around, or
she can/may choose him and have a very hard life.

I empathize with you, my daughter had the opportunity to live with me for free. Her education would be paid for as long as she had no babies
and no babies before marriage. That she did not overuse credit. When she graduated I would sell my home and give her 20 percent for her first home.

She agreed and then fell in LOVE, and 3 months later she
dropped out of school, left her job, and moved into bf parents house and then she wanted to get married!
No job, only 2 years of university, and her car died. He left njit university after 2 years, lives with parents, I said NO. Not now.
Because she wanted a baby, this was the reason behind the marriage thing. Her Step Dad and I live apart but we stood our ground.
As much as it hurt and it hurt a lot, we would not stand by her as she was destroying her potential.
We did not go to her wedding.
We did not talk for over a year. Today she and I are working on our relationship. Now if she wants to go back to school she has to pay for it.
She wants a baby, well they can't afford one! I told my daughter that she should not have a baby while living in another's house.
Adult children, they want....

It is good that you are close with her and your relationship good.
I know this will be a difficult for your daughter but she has to think like a mother, protect her child, and for her to understand this won't hurt forever.
Stand tall and an order of protection is the best thing for now. Stand your ground. I pray for you and your daughter.
Sincerely,
Happiedasiy