/u/traway9992226's advice is probably the best you can hope for. You sound like a good kid but there's still a lot about the world that you won't understand. What your dad did, in many states, would be considered statutory rape. It's a crime. Your dad was a predator. I know that's hard to hear but many criminals are someone's dad, mother, brother, sister, etc.

You can't change what your dad did, but when you're older you'll realize how fucked up it was. You'll understand your grandpa better. He's in a terrible situation. His minor daughter was groomed and raped. In order to see his daughter and grandchild, who he still loves dearly, he has to put up with the rapist's presence. That is not an easy situation. Time will not change how shitty your dad was. And one day, when you're older, you'll have to come to terms with what he did and what it means. Right now, you want it all to go away, and I understand that, because the drama would be stressful. But your dad is the cause of it, he created the situation and it can't be undone. It's not your grandfather's fault. He probably still interacts with your dad because he has to, because his seeing you and his daughter are depend on his inclusion. There's likely a lot going on behind the scenes that they shield you from.

The best advice you can get here is to let the adults work it out themselves. They may never get over this, they may never pat each other on the back and let bygones be bygones. You can't force people to move on. You won't be able to force it even when you're an adult. So you need to focus on your own life, doing well in school, learning as much as you can, and let them handle things.

Also, remember this - your perception of someone's relationship is just that, a perception. You don't know many of the details, because you're a kid and it's frankly not appropriate for you to know. At the end of the day, your dad groomed your mother and potentially perpetrated a crime. There was, and may still be, a power imbalance. You won't know any of that though, because you're a kid and you shouldn't, nor would you necessarily know what to look for. So give your grandad some credit, it's not your place to push him to let that go.