Family Estrangement - kids

I have a 2.5 year old boy and a 1 year old girl and I wanted to reach out for advice on how to talk to them about my estrangement with their grandfather, my dad.

I cut my father out of my life a couple years ago, but my only sibling, my sister still had a good relationship with him. She has four boys, including twins who are my sons age, so I know as they all get older their cousins are probably going to mention their grandpa, so I’m trying to figure out how to address. My father is a horrible person. He’s racist, homophobic, a misogynist, bigoted in every way and has been so ridiculously consumed by far right media for so many years that he’s just angry and hateful all the time and living in an alternate reality. I’ve confronted my sister about not only her relationship with him but that of our family’s as well, but they refuse to do anything about the situation.

What I’m worried about is the perception of grandpa by my kids’ cousins and that as they grow older they will try to convince them that “grandpa Daryl isn’t bad” and then my kids will wonder why they can’t have a relationship with him. I mean, once they’re 18 I will give them full rights to have whatever relationships they want with whomever they want, because they are adults at that point. But until then I feel like I have an obligation to protect them from him and his toxic behavior. He’s also violent and abusive, and has a history of abusing my mom and other family members. I assume I will never get the rest of my family on board with ostracizing and excommunicating him, but I know down the road it’s going to create a lot of questions from my kids that I’m just not sure how to address because my sister and her kids have a radically different perception of the person he is and they will inevitably communicate that to my kids. Appreciate any advice.