I would ban them both.
Our daughter is doing the same. She comes to the house(family) the boyfriend never comes. She has been hurt about it. But we spoke to her why. Once it is said, it is said, out in the open. She is in sin, scandal and Christ did say, it would be better to put mill stone around ones neck then to scandal these innocent ones.
By allowing her in the home, would be a sing that she is welcomed, as she is.
You have the right of what comes in and what goes out your door. Start now, you will have more to face as your family grows.