Help me out - I don't understand how someone WAS 'such a great dad' who would go to any lengths to see his kids, but is now someone who never sees them, and won't even send his daughter a birthday card.
What actually happened? His ex not agreeing to a certain pattern of contact could make things very hard, sure - but it sounds like HE has given up any effort to be in touch. As I'm sure you know, the idea that he can have no contact for years only to pick up at some point in the future after a divorce is gobsmacking nonsense. His kids might well not want to know him.
This is so sad. I have dad friends who have been through/are in nightmare contact situations with the children of former partners, having to deal with arrangements being cancelled at the last minute, drop offs being missed, ex moving dcs hundreds of miles away with no consultation - and the efforts they make to stay in touch with their kids are immense.
Is it really desire for your dd to know her half siblings that is driving this, or the fear that it could happen to you and her?
One more question! What does your dp say if you tell him that he not only SHOULD see his dcs, but that you find it shocking and disgraceful that he isn't making an effort? You have said this to him, right?