Oh dear.
Refusing to give contact details - unreasonable - court would almost certainly tell him to do so, solicitor will tell him he's being an arse...
Rejecting a (step) sibling - horrible, emotionally abusive to his blood children, needless to say. Presumably he was your eldest's 'dad' for a long while? That tells you all you need to know. How sad.
An eleven year old who already doesn't want to see him? Hmm. In a year or so, her wishes will be listened to. Silly, silly man...
And she doesn't want to see him because he's making her sleep in a conservatory, won't let her discuss her time there with her mum - ie, because he is abusive to her? Even sillier man...
Oh and to top it all he's threatened to jeopardise her schooling. Er, hello, does he actually want to end up in a contact centre?
I'd calmly inform your ex that if he wants to end up losing touch with his children, either because he's going to be assessed as abusing them OR because they will end up hating and despising him, he's going the right way about it. See the pro bono person, and applaud your H's stupidity in making his abuse so overt (refusing to bring back your child?!), talk to the lawyer about asking for a residency order so that you can put a stop to any 'not bringing the children back' nonsense, and until that is sorted, stop contact.
The biggest point here is that he can reject your eldest. I can't imagine the catastrophic effect that will be having not only on her/him, but on the other children. They've just learned that Daddy doesn't love unconditionally. He's just made the biggest mistake he's probably ever going to make in his life. When it bites, it'll be too late, I guess. So if it helps you, think about that. He has lost.