It sounds like this situation is a struggle for you. I hope you can continue to keep a calm head in the face of his behavior! I have a 18 month old with a similar person and he has used her and put her in potentially harmful situations, so i've been no contact. Worrying about your children's wellbeing can be stressful.
I've looked at a few articles and websites related to this and most of them talk about not talking bad about him with the children. One of the things I worry about eventually is that my daughter's father will start his manipulation with her. It's highly unlikely that he won't talk me down or devalue me with her. I want to encourage her to talk about what happens when she goes with him and if anything comes up that's a red flag, address it.
Other tactics to consider: Teach them to identify what they're feeling and that they have a right to feel that way and they should trust that and not change their mind if someone else tells them their feeling is wrong. I think pointing out what behavior is acceptable and what's not in everyday life. Lying is not acceptable, putting someone down is not acceptable, stuff like that. My daughter says cruel things to my son sometimes and I used to tell him to not let it bother him because she was hurting and she didn't mean it, but one time he told me I can't eat that because I'm fat! It was like a slap in the face to me. So I've been saying it's alright to feel angry, annoyed, frustrated etc, but hurting your brother is not ok. And I ask him to say how whatever comment made him feel and it wasn't ok. I also feel that learning to set boundaries is part of it, I don't know how that would look for kids.
Maybe also pointing out healthy relationships when you see them? I read that in regards to teaching your son what is a real man. In movies where a dad demonstrates good behavior bringing it to their attention. I guess whatever other things that will give them self-esteem, self-confidence and love for themselves. I'm planning to tell mine twice a day and maybe I'll ask them to repeat it lol "You are good, you are valuable, you are worthy" Maybe something about it's ok to be yourself, so they don't feel the need to be like him or try to meet his standards so he can look good. Hopefully that will be the only voice in their head they listen to.
I'd like to believe that if we give them the tools and knowledge they will realize who he is on their own. The thought of my baby asking me why does daddy do that,etc, etc....what do you do? Have your kids ever asked you things like that?
Anyhow, I hope whatever way you choose that your babies will be able to stay strong and confident no matter what happens. It's clear that you are committed to their emotional well-being so just keep on keepin on

I hope all the best for you and your girls.