Younger siblings situation

I don’t have a relationship with my dad. He’s always been in and out, controlling when he was in and just a mess overall. During his marriage to my mom he had 3 kids on the side. He never told anyone until he left my mom. My siblings are actually pretty big.. 16, 10 and 11. I’ve been trying to have a closer relationship with them but my dad has been telling them he doesn’t want them contacting me. Their mom is supportive & let’s them call me whenever they want. But my dad makes them block me when they go over to his house. It makes me really sad for them because I know exactly what they are going through but I can’t do anything since they’re minors. The main reason I don’t have a relationship with my dad is because he only talked to me when he needed money but when I was 12+ and needed food, clothes and even a place to live he would disappeared or said we had to figure it out..

I don’t know what to do. I’ve been crying since I last spoke to the oldest one. He really told her I was no good & I don’t care about them. They know it’s not true but still..