I’m so, so sorry for the stress and distress this is causing you. It must be wonderful to find someone to love again. It must be awful to be targeted on Facebook and to be so badly treated by girls you were prepared to love.
As difficult as it is, please don’t take their behavior personally. It’s probably not about you. You are the symbol that their parents are never going to get back together. They are loyal to their mom. They want life to go back to the way it was. As soon as you moved in with their dad, they knew that the breakup of the family was indeed permanent and they hate it.
You don’t have to be a “mom” to these girls. As you know, they have a mom and they don’t need another one. Hopefully, they will eventually let you be an adult friend. Meanwhile, all you can do is be yourself and stay cordial. You don’t need their approval.
What you do need is for their dad to step up and give you more active support. He may have “confronted” them with words but from the sounds of it, he’s a bit of a doormat with their behavior. There don’t seem to be any meaningful consequences for their disrespectful behavior.
How is it that the 16-year-old gets to come and go as she likes, regardless of her behavior? How is it that she’s allowed to keep a cellphone if she uses it to hurt you? It seems to me that he should be telling her that if she wants to visit, she needs to be at least polite. She doesn’t have to love you but she does need to treat you as she would any guest of his who is important to him. Ditto for the 18-year-old. How is it she just moved in? He can let her know in no uncertain terms that staying at his place means treating you well.
I hope you will insist, for your own sake and the sake of your daughter, that your boyfriend take charge of this situation before you marry him. There needs to be a significant change now or you will never feel welcome and at peace in your own home if they are around.
I wish you well.