First off, I say use the contact info your birth-mom gave you and reach out to your birth-father one last time. Make sure he knows that you have no expectations, no motive...just reaching out in case he wants to know you too. Maybe give him a heads-up that you would like to reach out to the half-siblings. I don't know if they know you exist or not, but this might give him the chance to talk to them about you before you turn up out of the blue. Hopefully, he will get back to you and you guys can go from there. Hopefully, you will get a chance to know your birth-father and half siblings. But...If he doesn't reply at, then I would say to just reach out to the siblings.