Is it too late?

Long story short.. my dad was an abusive cheater and my mom left him when I was about 4 and my sister was 2. Around that time, he also stopped picking us up for visits. He had a girlfriend that absolutely HATED us. Like HATED. He chose her over us and had 2 kids with her. I am almost 23, my sister is almost 21 and my dad has reached out. Over the last few years (that I know for sure.) He and his GF had a bad drug problem. They lost custody of their kids and my dad went to prison. Fast forward.. my dad has been sober for over 6 months, and has custody of his kids. He is no longer with that "woman". So like I said, he has been reaching out, apologizing and I just don't know.. I feel bad but its been over 15 years. I seen him here and there (by accident) as a child but still.. I don't know what to do. How do I go about having a relationship with him? Anyone ever gone through this.. I feel bad ignoring his messages but I am not ready..