I think most people here know that I have a horse in this race, but long story short: I have been an adult adoptee searching for family information, including submitting DNA to a service, but with no real results. I have given a lot of thought to what I would do in a situation like the half-siblings.

I have sympathy for the father; he had no more choice in the circumstances of the adoption than the adoptee did. And I don't think that the LW should push him to get involved in the half-sibling's life. But I also don't think that he has a right to deny his adult children contact with one another. Assuming that the half-sibling is open to contact (and think it's highly unlikely that an adult adoptee, especially one who was adopted when the vast majority of adoptions were closed, would submit DNA to a registry site without knowing that they might be connected with close biological relatives), the LW can develop that relationship without the father's involvement.

And I think their siblings have a right to make that decision for themselves as well. These are all people in their own right, not just extensions of the father, and they they had no say in the past, and should get to make their own decisions in the present.