AITA for refusing to take care of my deadbeat father's affair family after he's gone?

My deadbeat father walked out on me, my mom and my sister when I (21M) was 5 years old. He had a whole affair with this woman and he wanted to be with her. He had nothing to do with us, didn't even come to my sister's funeral two years later, never went to see her when she was in the hospital and never paid for a thing. He was resisting child support like crazy. When it became clear my deadbeat wasn't going to help at all and had no intention of even paying, his parents, who had a strained relationship after he left because they wanted her to stay open to taking him back, stepped up and provided for us when we needed it. They also said I would be the one to inherit when they passed. They had it set up from the time I was 10 years old. So my deadbeat gets nothing and the other kids he had with his affair partner get nothing either.

I knew when my deadbeat had other kids. I have never considered them siblings. My sister died when we were just kids and she will forever be my only real sibling. I grew up having zero to do with him or his affair family.

Then last year he reached out to me because he was diagnosed with cancer and wanted to reconcile. I refused to even answer him but he went to my grandparents and they told him I wanted nothing to do with him, not even on his deathbed. They hadn't had a relationship with him in a decade at that point but when they heard he was sick they did reply and part of them hoped it would mean he'd back off me.

This all leads me to the reason for my post and don't worry I won't drag it too long. But my deadbeat is now in the know about the inheritance stuff and the fact my grandparents have found a way that they can't sue for anything. But they get nothing from the estate. Even if my deadbeat were alive he'd get nothing but he'll be dead before them.

This made him reach out to me and he begged me to make sure his affair family are okay when he dies. He told me he and his affair partner made bad choices in the past and he wants to be sure the kids will be fine. I ignored him at first and then he pestered me about it so I replied that I want nothing to do with his affair family and won't help ever. He got hysterical via text and DM about this and told me his family are innocent and I should try to know them and help if they ever need it.

AITA?