What was your son's response?
My DS has always been one of those peope who is 1) very self contained 2) wise beyond his years. He got it. He understood b/c he trusts me. He and I are pretty tight. He knows he doesn't always agree with me but that I always have his back. I didn't go into detail about the things he NPD RB father did to me. Attempted murder is on the list. He, in fact, was convicted of it for trying to kill the woman who came after me. I did tell him about that. If you can find some extremely bad, documented things, that happened to a 3rd party maybe share that. I just didn't want it to be too real/close to home so I didn't tell him the things he did to me.
I also explained that I am sure that his father's behaviour were the result of childhood abuse. That his craziness wasn't something DS had to worry about inheriting. I educated him about NPD/BPD so he could understand more.
On a certain level he just wanted to hear things from me. I told him that I would 100% support him finding his biofather at age 18 and would help him. I asked him to trust that I have more life experience and therefore knew a little more about why DS needed to be older to be safe around NPD biofather.
Perhaps you could express your concerns about her being a bit too young to deal with this man. You could agree to consider it after she's had 6 months or a year of IC so she's more tools. Emphasize the tools that she needs, not that she needs to be fixed.
Its so scary. Lots of love to both of you. But that's just a lot of water
Underneath a bridge I burned
And there's no use in backtracking
Around corners I have turned
“Many of us crucify ourselves between two thieves - regret for the past and fear of the future.” -foulton oursler