Once a child becomes an adult, the sibling relationship can sometimes be more supportive and long-lasting than the parent-child relationship.
The link between your DD and her siblings is entirely separate to her relationship with her father. The one doesn't rely on her having a relationship with the other, though obviously it would be better if she did.
While your DD's dad sounds like an immature, irresponsible twat whose fertility far exceeds his morality, your DD still has a right to know the 50% of her heritage that hails from him. The chance to have a relationship with her siblings is part of that. Just as you love all of your DD and not just the 50% that she derives from you, your DDs siblings are all individuals in their own right and bear no responsibility for their father's actions nor any predisposition to behave like him.
Allowing the three of them to have a relationship could be a source of joy, support and broadened horizons for all of them and the only person who will ultimately miss out on all of this will be your DD's father. A child brought up in a fatherless but strong, supportive family network will be fine despite the absence of that father. The fact that your DD has a relationship with her eldest sister who also doesn't see her father will also actually help her to see that the problem lies with her father rather than with her. I certainly wouldn't ban the conversation of 'daddy' but let them work it out for themselves.
If you refuse to allow the relationships, however, you may well find your DD redirects the hurt and disappointment against her father right back at you, blaming you for the separation of her siblings (and possibly by extension with her father) simply because you're a more tangible target.
It's hard and messy and you're being understandably protective, but I really wouldn't stand in the way of this.